Badgers start to roam much further afield when culling starts nearby, research has found, potentially increasing the spread of bovine tuberculosis, the disease culling is meant to control.

The findings raise questions about the government’s culling strategy, begun in 2011 and intended to reduce the harm to dairy herds from a rising incidence of bovine TB in hotspots around the country. Last month the government announced a major extension.

However, farmers disputed the findings and the Department for Environment, Food and Rural Affairs (Defra) said the cull had been shown to be effective.

Surviving badgers in populations that were culled covered nearly two thirds more land each month than they did before the culling began, and the likelihood of a badger visiting neighbouring territories each night increased twentyfold, according to the study from the ZSL’s Institute of Zoology. The increase in badgers’ range came despite the animals being less likely to leave their setts overall in the aftermath of a cull, making them less visible to the marksmen carrying out the culls.

The study, published in the Journal of Applied Ecology, covered 67 badgers on 20 cattle farms in areas of Cornwall both with and without culling, from 2013 to 2017.

Lord Krebs, whose landmark review of the science of bovine TB in the 1990s resulted in a 10-year trial of culls, said: “This research shows how important it is to find out about badger behaviour. It shows that culling badgers can cause surviving individuals in an area to move around more, and as a result they could come into contact with infected cattle and help to spread TB. The ill-thought-out plan to control TB by killing badgers could therefore backfire.”

The National Farmers’ Union said the ZSL study was small and covered only one county, and that it was “vital” the cull should continue to be rolled out across the country.

Stuart Roberts, the vice president of the NFU, said: “Culling badgers has a proven impact on TB outbreaks among cattle – the aim must be to get rid of this awful disease. Previously published peer-reviewed research, and anecdotal evidence from farmers in these areas, indicates strongly that TB is being reduced as a result of controlling the wildlife which carry and spread the disease.”

He added: “We do not see similar convincing outcomes from vaccination. Vaccination may have a role to play in areas where TB hasn’t taken hold but it cannot cure a sick badger so, in areas where TB is endemic, culling is vital.”

Nearly 33,000 cattle were slaughtered in England last year because of bovine TB, and 3,600 farms were newly affected by the disease, according to the NFU.

Bovine TB is the greatest threat to animal health in the UK, according to Defra. A spokesperson said the culling policy already took into account the potential increase in badger movement, by intensively culling across a given area, and that research showed it was effective. “Bovine TB [costs] taxpayers over £100m a year and causes devastation and distress for farmers and rural communities,” they said. “There is no single measure that will provide an easy answer and we are pursuing a range of interventions to eradicate it by 2038, including tighter cattle movement controls, regular testing and vaccinations.”

Defra’s research has found a 58% reduction of bovine TB in the area culled in Gloucestershire, compared with unculled areas, while there was a reduction of a fifth in Somerset, after two years.

TB bacteria can remain viable over long periods in the environment, meaning that if infected badgers are roaming further after a cull, as ZSL found, this could create new sources of infection for months afterwards. The changes in behaviour were noted by the researchers as soon as the cull started in a given population, meaning even badgers that are subsequently killed in the cull could have spread the disease much further than if they had remained undisturbed.

Arthur Thomas, the campaigns manager at the International Fund for Animal Welfare, said: “This report raises serious concerns about the effectiveness of the badger cull policy. We believe that the evidence against the culling of badgers as a response to bovine TB is well established and overwhelming. Government must act quickly to put a stop to an ineffective and inhumane practice.”