A former government chief vet and former government adviser on culling are also among those urging the prime minister to intervene.
In a letter sent today, seen by The Independent, the 23 lobbyists remind Mr Johnson of a government promise earlier this year to phase out intensive culling and instead move to badger vaccination.
The experts, including Green MP Caroline Lucas and broadcaster Chris Packham, ask Mr Johnson to honour those words and step in to stop the expansion.
They warn him: “If, however, your government chooses to continue the discredited and ineffective badger culling policy, you will be remembered as the prime minister who presided over the greatest slaughter of a protected animal in living memory.”
Last month, leaked documents suggested the badger cull, which was begun in an attempt to stop dairy cattle from catching tuberculosis (bTB) from badgers, would be expanded to 11 new areas, bringing the total this year to 54.
The widened operation, due to begin this month, would kill 65,000 badgers, conservationists estimate.
They include for the first time Derbyshire, Oxfordshire, Warwickshire, Shropshire, Leicestershire and Lincolnshire, which is considered a low-risk TB area.
Derbyshire has the biggest badger vaccination project in the country, and an expansion would mean even inoculated animals could be killed.
The letter is also signed by Ranald Munro, the government-appointed former chairman of an independent panel examining culling, former government chief vet Alick Simmons and Will Travers, executive director of Born Free.
It argues that culling is ineffective in curbing bTB. The prevalence of the disease in cattle herds in Gloucestershire and Dorset was higher following pilot culling than before, it claims.
In Cumbria, only three badgers out of 317 killed last year tested positive, the letter adds, also citing evidence that the cattle skin test is ineffective.
“We applaud your government’s stated aim of phasing out badger culling, but this appears to be in stark contrast to your apparent intention,” the experts write.
And they add that public opinion will be with Mr Johnson if he halts the expanded cull.
Since 2013, more than 100,000 badgers have been killed in England.
The NFU last year cited research, published in Scientific Reports, that “showed a 66 per cent reduction in new TB breakdowns in cattle in Gloucestershire and a 37 per cent reduction in Somerset”.
The Department for Environment, Food and Rural Affairs, said recently: “Bovine TB remains the greatest animal health threat to the UK, costing taxpayers over £100m every year as well as causing devastation and distress for farmers and rural communities.
“There is no single measure that will provide an easy answer to beating the disease and we are pursuing a range of interventions to eradicate it by 2038, including tighter cattle movement controls, regular testing and vaccinations.”
The Independent has asked Downing Street to respond.
