Opponents of the badger cull in Gloucestershire are mounting a legal challenge to halt an extension of the controversial pilot, it has emerged.
The Badger Trust has formally warned the authorities to abandon the shooting or face a High Court challenge on the grounds that a plan to more than double the killing period is illegal.
The six-week pilot in the west of the county killed 708 of an estimated 2,350 badgers - about 30 per cent of the total, rather than the planned 70 per cent.
And in west Somerset 850 badgers had been culled during the pilot - 60 per cent of the local population.
Natural England, which licenses the pilots, is considering an eight-week extension application for Gloucestershire after earlier granting an extra three weeks in Somerset.
The Badger Trust has issued a "pre-action protocol letter" to Natural England and the Treasury Solicitor for the Secretary of State Secretary of State for the Department for Environment, Food and Rural Affairs, which is the first stage in seeking a judicial review in the High Court.
The lengthy letter, from law firm Bindmans LLP, quotes ministers' and officials' own words on the importance of restricting the cull to six weeks.
"We consider that the decision to extend the cull in Somerset was unlawful, particularly in the light of those documents," the letter states.
"But given that we have only just seen them and given the relatively shorter extension there, we have advised our clients that it would not now be practicable (given the court process) to seek injunctive relief to prevent it.
"But that is not the case in relation to the contemplated decision in relation to Gloucestershire not least because of the much longer extension (eight weeks) being sought (and not yet granted) and the fact that our clients have now seen the nature of the arguments being relied on to justify further culling."
The controversial pilot projects have been launched in Gloucestershire and neighbouring Somerset, in the face of widespread protests, in an effort to control the spread of bovine tuberculosis which is carried by badgers.
Defra Secretary of State Owen Paterson told the House of Commons this week that early indications showed the culls in both counties were carried out in a "safe and humane" way, but had demonstrated that "the cull period may need to be longer than six weeks in future, enabling teams to adapt their approaches to suit local circumstances".
Mr Paterson confirmed that the authorities are considering whether to try gassing badgers, but said "we will not deploy any method of removing wildlife which is not safe, humane and effective".
He added: "I have to remind people that up to the end of July a further 20,000 perfectly healthy cattle have been hauled off to slaughter at horrendous expense to the taxpayer because we've lost control of TB. We've lost 305,000 cattle over the last 10 years.
"It's not acceptable to me to allow this disease to go on when we know that other countries - Australia, New Zealand, the United States, the Republic of Ireland - where they've borne down on the disease in cattle and they've borne down on the disease in wildlife."
Campaigners against the cull programme branded it a "catalogue of failures" and urged the Government to admit the policy was now "in tatters" and start vaccinating badgers.
Jeanne Berry, from Gloucestershire Against Badger Shooting, said: "It is clear that the Government did not do the maths before the cull and now they are now clutching at straws by attempting to kill more badgers in three weeks than they did in six weeks.
"Extending the cull will, as scientists have confirmed, increase the spread of bovine TB and of course increase further police costs, which are now over £1,400 a badger.
"We urge the Government to change course and take up badger vaccination instead."