A fierce landmark court battle over the badger cull begins tomorrow as wildlife campaigners mount a challenge to the reasoning behind the killing of tens of thousands of the mammals.

If a judge rules in the campaigners’ favour, the cull licences will be quashed, forcing the government to rethink its policy affecting huge numbers of animals and farmers.

Cull opponents have already accused government ministers and officials of “incompetence, negligence and deceit” in drawing up their plans and extending the “growing series of brutal, bloody countryside purges”.

And they warn that a flawed cull could worsen the spread of bovine tuberculosis (bTB) rather than alleviate it.

If licences were quashed this week ministers could have to launch a fresh consultation over culling, providing data that was “omitted” last time.

Since culling began in 2013 in an attempt to control tuberculosis in cattle, more than 30,000 badgers have been shot. In three months in 2016 alone, 10,000 were culled. Opponents have called it “the biggest slaughter of a protected species in living memory”.

Despite this, the disease is still rising in England, leading experts to conclude there is no evidence that culling contains it.

But environment secretary Michael Gove has agreed a further expansion of the cull zones this summer, which wildlife experts say could result in about 120,000 badgers being killed by 2020, and up to half a million by 2028.

Last year 19,274 badgers were culled by shooting, government figures show, out of a population thought to be between 100,000 and 200,000, although some estimates have put it at as high as 550,000 including young cubs.

In addition, at least 43,000 cattle were slaughtered because of the disease last year – a rise of 4,000 on 2016, Environment Department (Defra) figures show.

This week’s judicial review – launched by ecologist and Badger Trust member Tom Langton, who believes officials acted unlawfully – challenges two aspects of the cull policy.

The first is over the way in which Natural England, the government body in charge of protecting wildlife, did its homework before issuing licences to shoot badgers. Mr Langton and the trust claim its habitats regulations assessments, including judging the effect on protected species such as hen harriers, were flawed.

The second challenge is over a consultation in 2016-17 and the decision last summer by Department of Environment officials to grant more culling licences in parts of Gloucestershire and Somerset for a further five years. The cull opponents say the officials wrongly interpreted the conclusions of randomised culling trials.

Among seven volumes of documentation, the Badger Trust will tell the High Court that former environment secretary Andrea Leadsom, her successor Michael Gove and ultimately Theresa May approved supplementary cull licences “without any scientific justification, proper consultation or adequate environmental assessments”.

Documents that Natural England released about its research were heavily redacted, a move condemned by the Information Commissioner.

“The shocking revelations about what’s been going on behind closed doors under a cloak of secrecy created by the government under a facade off public safety will be in the spotlight,” Mr Langton told The Independent.

“If they’ve got the bTB policy wrong and are doing more harm than good they’re in breach of regulations.”

Dominic Dyer, head of the trust, said: “This has led to badgers being killed in sensitive wildlife habitats without essential precautionary measures required under EU and British habitat protection regulations and legislation.”

A judicial review is a challenge to the way in which a public body arrived at its conclusions, rather than the conclusions themselves.

Mr Langton will provide evidence that he says shows the proper processes were not followed to reach the decision to expand the cull.

He said: “Senior government officials have operated in a manner that we believe is unlawful.

“The government has moved from attempting a precision badger removal policy to an open-ended badger eradication approach that has no scientific validity and that independent experts believe could easily do more harm than good.

“Further, no serious efforts have been made to consider the ecological impact of widespread badger removal from the countryside, particularly in relation to the impact of predator changes on sensitive wildlife habitats and species including rare birds.

“This case is an important fight not just for the badger, but also for the future of our countryside and the farming industry.

“The badger cull policy is failing farmers, taxpayers and our precious wildlife, and will make the bovine TB epidemic worse.”

The Zoological Society of London has warned the supplementary culls could do more harm than good by disrupting populations of the nocturnal mammal, moving the disease to fresh areas.

The trust says it also contravenes the Protection of Badgers Act 1992 and could lead to wildlife habitats of international importance being threatened.

Mr Dyer added: “The government has no credibility left when it comes to the disastrous badger cull policy.

“The High Court will hear evidence that Andrea Leadsom and Michael Gove lacked proper scientific evidence and overstepped their positions of power and influence.”

The planned cull expansion will allow permits to be issued in low-risk areas in up to 10 zones a year for three years if there is an outbreak.

We aren’t just helping badgers, but also farmers and the dairy industry Badger Trust member Tom Langton

The Badger Trust estimates the cost of the cull to the taxpayer across the 21 areas now involved is more than £50m, with the cost of policing the cull in Cheshire alone last autumn approaching £1m.

The campaigners say wildlife becomes infected when bovine TB spills out into their populations, and wants it tackled “at source” through better testing with new technology, movement controls, vaccinations and new quarantine measures.

Mr Langton said he was hopeful but the outcome of the hearing – due to last four days, with a judgment due later this summer – was finely balanced.

“We aren’t just helping badgers, but also farmers and the dairy industry because the policy we’re pursuing is the equivalent of sailing out of port not knowing whether the bow doors are open, and not being able to check,” he told The Independent.

A spokesman for Defra said: “Bovine TB is a slow-moving, insidious disease which presents many challenges. It is difficult to detect and there’s no single measure that will provide an easy answer.

"That is why we are pursuing a wide range of interventions, including cattle-movement controls and a cull of badgers where they are linked with herd breakdowns.