An independent study of pilot badger culls is said to have concluded that they were ineffective and too many animals suffered.
Research commissioned by the Government found that the number of badgers killed in Somerset and Gloucestershire fell short of targets set to limit the spread of TB in cattle.
More than 5 per cent of badgers took longer than five minutes to die, failing the test for humaneness, according to the BBC.
The Independent Expert Panel was appointed by the Department for Environment and Rural Affairs (Defra) to evaluate the pilots.
Marksmen were brought in to shoot the animals at night, with the aim of killing at least 70% in cull areas within a six-week period.
The duration was extended after initial figures suggested that just 58 per cent of badgers were eradicated in the Somerset scheme and 30 per cent in Gloucestershire.
However, the independent panel is reported to have found that less than half of badgers were killed in both areas during the first six weeks.
Defra had also agreed that the standard for declaring the culls humane would be 95 per cent of the shot badgers dying within five minutes.
But the expert group has apparently concluded that between 6.4 per cent and 18 per cent of animals exceeded that limit, depending on the assumptions made.
No date has yet been set for the report to be published.
Leading activist Brian May said: "We should also mention that the report discovered that it was very inhumane as well. I don't think people will stand for this. You're talking about badgers taking five or 10 minutes to die.
"Owen Paterson's denied that but it's obviously true."
The guitarist with rock band Queen told BBC Breakfast that he had "a lot of sympathy for farmers", but added: "This is not the way to solve the problem. The way we believe we can solve it is by vaccinating the badgers, and also vaccinating the cows."
May said badgers can be vaccinated for "about £120 a head", and added that it has "just cost £4,200 per badger to kill the poor things".
Robin Hargreaves, president of the British Veterinary Association (BVA), said: "It is important to remember that these culls were pilots precisely because the Government needed to test the humaneness, safety and efficacy of controlled shooting as a method of culling badgers.
"Indeed, BVA called for controlled shooting to be tested and critically evaluated before it was rolled out.
"We are unable to comment in detail on the findings of the IEP until we have seen the report and had time to review all of the information but, if these figures are true, then they would certainly raise concerns about both the humaneness and efficacy of controlled shooting.
"We will look at the published report in detail and consider BVA's position in light of the IEP's findings.
"We have always stated that if the pilots were to fail on humaneness then BVA could not support the wider roll-out of the method of controlled shooting."
Dominic Dyer, chief executive of the Badger Trust and policy adviser to animal charity Care For The Wild, which has campaigned against the cull, said: "Anyone who has scrutinised the disastrous badger culls will have hoped for, and expected, an independent report to come up with this result - that the culls failed completely on both efficiency and humaneness grounds."
He called on Prime Minister David Cameron to intervene to stop the cull, which he said had been introduced for political, not practical, reasons.
He said the pilot culls had probably cost more than £10 million, or £4,000 per badger, and could increase the risk of TB in cattle as badgers moved around more following culling. Shooting of free-running badgers in the cull had been shown to be a failure, he added.
He also said many badgers could have been left to die long painful deaths as a result of a lack of effective monitoring for humaneness.
"The badger cull pilots have been a complete failure on scientific, economic and animal welfare grounds, and now the independent expert panel has reached this conclusion, all future plans to cull badgers should be stopped immediately," he said.
A spokesman for the Environment Department (Defra) said: "The Independent Expert Panel has not submitted its report to ministers and the report has not been published.
"We knew there'd be lessons to be learned from the first year of the pilot culls which is why we're looking forward to receiving the panel's recommendations for improving the way they are carried out, because we need to do all we can to tackle this devastating disease."
The panel was appointed to look at the effectiveness, humaneness and safety of "controlled shooting" - shooting free running badgers, as opposed to those which are cage-trapped and then shot - as a culling method.
A Freedom of Information request revealed that only 24 per cent of the badgers estimated to be in the two pilot areas were killed by controlled shooting in the original six-week period of the cull.
Farmers and the Government insist a cull of badgers, which can spread TB to cattle, is necessary as part of a package of measures to tackle the disease in livestock.
But opponents of the cull have claimed for years that it would be ineffective and inhumane and have called for tighter measures on farms and vaccination of both badgers and cattle to tackle TB.
Wildlife film-maker and presenter Simon King, the Wildlife Trusts' president, said: "The culls were flawed from the beginning and this seems to be concrete proof. We look forward to the cessation of all rhetoric that culling should continue.
"We need to deal with bovine tuberculosis in a practical and meaningful way to support the farming community."
Green Party MP Caroline Lucas said: "Now that its own research has demonstrated that culling is cruel as well as pointless, it's time for the Government to heed the evidence and end this failed policy once and for all.
"Scientists have long been telling us that culling was going to fail on grounds of both humaneness and effectiveness.
"Bovine TB is a serious problem and we need new, effective ways of dealing with it.
"Strategies based on vaccination, improved biosecurity and stricter controls on the movement of cattle were always going to offer more hope than culling, which has caused unnecessary suffering to animals, as well as wasting huge amounts of public money."
Monitoring by government agency Natural England, which has been released following a Freedom of Information request, revealed that some badgers were shot in the head, neck and shoulder, against best practice, or took more than one shot to be killed.
In one instance among 41 visits to check that those carrying out the cull were adhering to their licence conditions, it took five to 10 minutes to administer the second, fatal, shot to a wounded badger.
In some cases contractors did not take biosecurity precautions such as wearing masks or gloves, or using disinfectant.
Veterinarian Mark Jones, executive director of the Humane Society International UK, said: "While the level of observation of controlled shoots was clearly inadequate, if the proportion of poor shot placement and wounding observed is in any way representative of the shooting as a whole, we could potentially be looking at hundreds of badgers enduring pain and distress.
"Indeed, if wounding, incorrect kill shots and biosecurity breaches were recorded when the shooters were being monitored and arguably on their best behaviour, it is quite possible that even more unprofessional conduct and animal suffering could have occurred when no-one was watching.
"Defra's myth of a humane and professional cull has been exposed. It would be a travesty to allow this unjustified slaughter of badgers to continue.
"We've always suspected that the cull was inhumane, now we have evidence to show it. It's time to kill the cull."
Shadow environment secretary Maria Eagle said: "Ministers must not ignore this damning verdict on the failure of the Government's badger culls. This independent report looks set to confirm that less than half of the local badger population was removed in each pilot area, despite ministers previously accepting that 70% was the minimum to be effective and not risk the further spread of TB through perturbation.
"Appallingly, the analysis of the pilots has apparently also found that they failed the Government's own 'humaneness' test.
"It would be outrageous if ministers now sought to dismiss the advice of their own Independent Expert Panel and press on with further culling of badgers regardless. The Environment Secretary should come back to Parliament on this issue and there should be no roll-out of the Government's badger cull policy without a full debate and vote in Parliament.
"These culls have been a disaster for taxpayers, farmers and wildlife. The Government must now put scientific evidence back at the heart of their approach to the serious issue of bovine TB, instead of being led by the dogmatic personal prejudices of ministers.
"They should agree to cross-party talks with the aim of securing a renewed consensus for the long term on eradicating bovine TB through an alternative strategy based on vaccination and tougher restrictions on cattle movement."