Campaigners have reacted angrily after it emerged that badger culls could be extended by three weeks because not enough animals have been killed.

The culls in west Somerset and Gloucestershire could be extended to maximise the impact on the spread of bovine tuberculosis, Environment Secretary Owen Paterson revealed.

Mr Paterson said the six-week action in west Somerset has killed 850 badgers - 60% of the local population.

"Current indications suggest that the pilot has been safe, humane and effective in delivering a reduction in the badger population of just under 60%.

"We set ourselves a challenging target of aiming to ensure that 70% of the badger population was removed during the pilot. The chief veterinary officer (CVO) has advised that the 60% reduction this year will deliver clear disease benefits as part of a four-year cull.

"However, Natural England are considering an application from Somerset for a short extension of the culling period, as provided for under the agreement with the company.

"The advice of the CVO is that further increasing the number of badgers culled would improve those benefits even further and enable them to accrue earlier."

He added in a statement to MPs : "The cull in Gloucestershire is still ongoing and I will make a further statement when the six weeks is completed. I understand that this morning Gloucestershire is also submitting an application for an extension to Natural England."

Mr Paterson indicated that in future the period for culls may have to be longer than six weeks to ensure "high levels of safety and humaneness".

"This Government is committed to tackling the disease in all reservoirs and by all available means. Our cattle industry and the countryside deserve no less," he added.

But Queen guitarist Brian May, a leading opponent of the cull, said figures showing the number of badgers killed were "widely disparate", and urged ministers to abandon the "ridiculous" cull.

He told BBC Breakfast: "To be honest, it's getting more and more ridiculous. This is the third lot of figures we've seen, they're widely disparate.

"They have no idea how many badgers there are and they keep adjusting the figures to make it look like this is a success.

"This was supposed to be a two-week shoot. It's already been extended to six weeks from two - now they want another three weeks.

"This has demonstrated already that free shooting doesn't work. The only way they got to 800 badgers was by cage-trapping and shooting.

"If you are cage-trapping and shooting, you can cage-trap and vaccinate.

"There is no way on earth that culling badgers can lead to the eradication of TB.

"We have to abandon this ridiculous cull.

"This cull is a waste of time and a waste of our money."

Anti-cull campaigners branded the Government announcement as either "suspiciously convenient or dangerously incompetent".

Wendy Higgins, communications director for Humane Society International UK, said: "It is suspiciously convenient that, as Defra ministers were staring down the barrel of an unmitigated disaster, the badger kill targets have been halved and the Government will declare the cull a success when everyone knows it's been an utter shambles.

"Either that, or Defra has been dangerously incompetent and shown itself incapable of even counting badgers.

"Either way, it is truly shocking that the kill targets for shooters were set higher than the actual number of badgers in the cull zone, and it is only the shooters' inability to meet those targets that prevented local badger populations from being wiped out altogether.

"If that had happened, Britain would now be in breach of the Bern Convention, which is why HSI UK appealed to Bern to stop the cull last year.

"In light of this, it is quite outrageous that Defra is seeking to extend the cruelty and chaos for another three weeks. This has got to stop. Defra needs to come clean and kill the cull for good."

Meanwhile, the NFU welcomed Mr Paterson's written statement.

President Peter Kendall said: "After the Secretary of State's comments today on the progress that has been made, I want to thank those involved in carrying out what is a very important first step on the long road towards eradicating TB in cattle, in badgers and from our countryside," he said.

"Safety and humaneness are two really important tests.

"I am also pleased to hear confirmation from the Government Chief Vet that the current cull operations in Somerset to date will deliver disease reduction as part of a four-year plan.

"I understand that the company carrying out the cull in Somerset has applied to Natural England to extend the culling period to enhance its disease control.

"They made this application towards the end of the six-week pilot cull and I understand that a decision will be taken this week whether to extend the licence.

"The knowledge learned from these two badger cull pilot areas will be invaluable in helping to deliver future roll-out of badger control operations in areas where the incidence of TB is rife.

"Our absolute focus, and that of everyone involved, is disease control. More than 38,000 cattle were slaughtered in Great Britain in 2012 because of bovine TB.

"These badger cull pilots are a very important first step in what is a 25-year strategy to eradicate this terrible and infectious disease."

Asked on BBC's Spotlight programme in the West Country whether he was "moving the goalposts", Mr Paterson said: "That's not right, the badgers have moved the goalposts."

He added: "You are dealing with wild animals. It is a wild animal subject to the vagaries of the weather, disease and breeding patterns."

Labour's shadow environment secretary Maria Eagle said: "The extension of these badger cull trials demonstrates that the Government's approach is not working, which is hardly surprising when it was not based on any scientific evidence. There is now a real danger that even longer trials could exacerbate spread of TB as more badgers flee, risking infecting cattle in other areas.

"Eradicating TB from cattle is vital for farmers, the wider rural economy, taxpayers and wildlife which is why we need a science-based approach focused on vaccination of cattle and badgers and proper restrictions on the movement of livestock.

"Instead of going to ground, Owen Paterson should have immediately come before Parliament to explain what has gone wrong with his badger cull."

Joe Duckworth, chief executive of the League Against Cruel Sports said: " We are astounded at the incompetence of the Government on this matter. The cull was postponed last year due to errors on badger numbers, but yet again we are in the same position of confusion on the basics. The whole policy flies in the face of scientific sensibility.

"The Government is opening itself up to accusations of 'fixing' the cull results for their own political ends."

Mr May branded the badger cull an "utter failure" and described the application for an extension as a "farce".

"Peter Kendall trying to pretend it's a success by changing the initial estimate of numbers is ludicrous," he said.

"It's a failure because they said they had to cull 70% and they failed to do that. They are now applying for an extension.

"This is becoming a farce. They are now being told that probably the prevalence (of TB) has increased in badgers already.

"They were warned this would happen and they did not listen to the scientists. Pretending this is a science-based cull is a farce.

"Nobody knows how many badgers there are out there. There was all this talk of an explosion in badger population. If this latest figure is to be believed, there is absolutely no explosion whatsoever.

"How can they keep adjusting these figures? What's the basis for what they are saying?

"The fact they only got 200 badgers after the first couple of weeks is very significant because it shows free shooting does not work as a system.

"The only reason they got 800 badgers is because they went over to trapping badgers and killing them.

"If you can trap a badger and shoot it in the head, you can trap a badger and vaccinate it for the same amount of money.

"The whole argument for this cull has been 'We have to do something now and it has to be affordable'.

"Vaccination is very affordable. I heard a figure the other day of £85,000 per badger killed. This is ludicrous and is our money being spent and it will not solve the problem.

"It's a waste of our money and it's incredibly inhumane to the badgers, to the cows and to the farmers."

May said the application to extend the pilot should be refused and the cull should not be rolled out across the country.

"Instead of wasting our energy with this war that is going on, we should be getting together as we are in Wales to vaccinate the population of badgers and in a few years take them out of the equation," he said.

"We should also move fast on Europe. Fast will mean 10 years but compared to 25 years of culling with Owen Paterson's scheme, that is a pretty good deal."