Pakistan is actively helping the Taliban retake control of Afghanistan, a leaked Nato report has concluded.

The top secret document seen by the BBC and The Times sets out the Western alliance's belief that the Pakistani intelligence agency, the ISI, is "intimately involved" in the fight against coalition troops in the country. It says the Taliban is poised to retake the country once Nato departs.

"ISI is thoroughly aware of Taliban activities and the whereabouts of all senior Taliban personnel. Senior Taliban leaders meet regularly with ISI personnel, who advise on strategy and relay any pertinent concerns of the government of Pakistan," the report is quoted as saying.

The document is reportedly based on interrogations with more than 4,000 captured Taliban and al-Qaeda operatives at the US Bagram airbase in Afghanistan.

It quotes a senior al-Qaeda detainee as saying: "Pakistan knows everything. They control everything. I can't [expletive] on a tree in Kunar without them watching." The prisoner adds: "The Taliban are not Islam. The Taliban are Islamabad."

Relations between Pakistan at Nato have been increasingly strained in recent months, but it has long denied it has helped direct attacks against Nato forces in Afghanistan.

On Tuesday President Obama said controversial drone strikes inside Pakistan, which Islamabad views a violation of its sovereignty, targeted "people who are on a list of active terrorists".

A cross-border Nato air attack that killed 24 Pakistani soldiers in November severely damaged relations and led Pakistan to close supply routes to Western forces in Afghanistan.

The report also concludes that many Afghans expect the Taliban to return to power within years. "Once Isaf is no longer a factor, Taliban consider their victory inevitable," it warns.

A Taliban spokesman quoted by Reuters said the insurgents would not countenance a cease-fire with Nato in order to begin peace talks.

"Our struggle and jihad will continue until we have installed a complete Islamic government in Afghanistan, regardless of the year 2014 or 2015 when the foreign troops say will leave Afghanistan," the spokesman said.

David Cameron has said he intends to withdraw all British combat forces from Afghanistan by the end of 2014. While France has decided to leave the country a year earlier than planned in 2013.

On Monday Afghan president Hamid Karzai visited London and signed an agreement that will see Britain build an officer training college in Afghanistan, modelled on the British Army's Sandhurst Academy.

Former Lib Dem leader Menzies Campbell said the report, if accurate, underlined "how difficult the task is in Afghanistan for the forces that are there".

He told the Huffington Post UK that it was a mistake for the West to view the ISI as an integrated structure.  "I don't think that is the case," he said.

"The ISI operates at very different levels. It may be when a senior figure in the ISI says they are not co-operating with the Taliban, that does not mean to say that lower down the food chain in the ISI there may not be individuals co-operating with the Taliban."