Sir David Attenborough has condemned the UK government for "ignoring" scientific evidence after the controversial badger cull in Gloucestershire was more than doubled in length on Wednesday.

The night-time shoots had killed less than half the minimum numbers required in the initial six weeks, prompting experts to warn than the failure risked increasing tuberculosis in cattle, rather than curbing it.

"Why do they spend a lot of time and money doing careful scientific studies and then simply ignore the results?" asked Sir David, the UK's best known naturalist. He told the Guardian: "They decided to have a six-week [cull] and when they don't get the result they want, they want to extend by eight weeks. It is simply not believing in the science."

The granting of the extension by Natural England (NE) will be a relief to the environment secretary, Owen Paterson, and the National Farmers Union (NFU), who argue that culling badgers is an essential part of curbing the rise in bovine TB, which led to 28,000 cattle being slaughtered in 2012 at a cost to taxpayers of £100m.

But Paterson now faces the prospect of a high court legal challenge from the Badger Trust and the disapproval of the nation's largest private landowner, the National Trust. Dame Helen Ghosh, the trust's director general and until 2010 permanent secretary at the Department for Environment, Food and Rural Affairs (Defra), had questioned the "scientific rigour and credibility" of the pilot culls in Gloucestershire and Somerset.

On Wednesday, the Guardian revealed that police in the two counties have received dozens of reports of illegal shooting, poisoning and gassing of badgers and sett interference in the last 15 months. Experts said illegal culling and problems with population estimates are more likely explanations for the steep drop in the government's estimates of badger populations than the "weather and disease" cited by Paterson.

The eight-week extension to the Gloucestershire cull follows a three-week extension in Somerset. Natural England said the Gloucestershire decision was made with "regard to guidance from government and supplementary advice from Defra's chief veterinary officer and chief scientific adviser".

But the lead scientist on the non-executive board of Natural England, badger expert Prof David Macdonald, said his personal decision would have been to halt the culling now. He told the Guardian: "I have little confidence this process is going to deliver the longed-for benefits for farmers in TB control. It is important people understand that NE is a statutory body working in a framework set by government. It was not asked the question: 'do you think this is a great idea'? "

A Defra spokesman said: "Natural England's decision will enable the cull company to remove more badgers and make a bigger contribution to reducing this terrible disease that is causing misery for farmers in the area. This follows the advice from the chief vet that if more badgers are removed in the first year of the cull, we will see greater and quicker reductions in TB in that area."

Andy Robertson, NFU director general, said: "Disease control has always been the absolute focus for everyone involved with the culls. The chief vet said that extending the culling period would help achieve the earliest and greatest possible impact on bovine TB in Gloucestershire so we welcome Natural England's decision.

Prof Rosie Woodroffe, a key member of an earlier, decade-long culling trial said: "The chief vet's advice to carry on culling is based upon a very incomplete view of the available evidence. Pushing through this cull extension before the government's own independent expert panel has had a chance to assess the pilots shows just how little Defra seems to care about scientific evidence on this matter. TB is a huge problem for cattle farmers, and they deserve better than to be left footing the bill for culling when the likely benefits are diminishing and the costs are spiralling."

Cull opponents reacted angrily to the cull extension. Mark Jones, executive director of the Humane Society International UK, said: "I am appalled and flabbergasted. The recommendations of [decade-long] trial were very clear – the longer you subject badgers to this sort of disruption, the greater the risk of worsening the spread of bovine TB among both badgers and cattle."

Patrick Begg, from the National Trust, said: "Our concerns remain about the utility of the scientific evidence that can be made from the cull pilots. Changing the original design to such a radical degree cannot be helpful in the important analysis that follows."