Agents from MI6 entered secret talks with Taliban leaders despite Gordon Brown's pledge that Britain would not negotiate with terrorists, The Daily Telegraph can disclose.

Officers from the Secret Intelligence Service staged discussions, known as "jirgas", with senior insurgents on several occasions over the summer.

An intelligence source said: "The SIS officers were understood to have sought peace directly with the Taliban with them coming across as some sort of armed militia. The British would also provide 'mentoring' for the Taliban."

The disclosure comes only a fortnight after the Prime Minister told the House of Commons: "We will not enter into any negotiations with these people."

Opposition leaders said that Mr Brown had "some explaining to do".

The Government was apparently prepared to admit that the talks had taken place but Gordon Brown was thought to have "bottled out" just before Prime Minister's Questions on Dec 12, when he made his denial instead.

It is thought that the Americans were extremely unhappy with the news becoming public that an ally was negotiating with terrorists who supported the September 11 attackers.

The delicate balance in Afghanistan was underlined as it emerged that two diplomats had been ordered by the Kabul government to leave the country after allegations that they had met Taliban insurgents without the administration's knowledge.

The pair, a top European Union official and a United Nations staff member, were declared "persona non grata" and said to be "threatening national security".

They are both Afghan experts who have been working in the country since the 1980s. They are in their forties and cannot be named. One man works as a political adviser to the European Union while the other is employed as a political adviser to the UN mission in Kabul.

One of the men described the charges as "banal and preposterous" and said he hoped the Afghan government would quickly drop its threat to deport them.

MI6's meetings with the Taliban took place up to half a dozen times at houses on the outskirts of Lashkah Gah and in villages in the Upper Gereshk valley, to the north-east of Helmand's main town.

The compounds were surrounded by a force of British infantry providing a security cordon.

To maintain the stance that President Hamid Karzai's government was leading the negotiations the clandestine meetings took place in the presence of Afghan officials.

"These meetings were with up to a dozen Taliban or with Taliban who had only recently laid down their arms," an intelligence source said. "The impression was that these were important motivating figures inside the Taliban."

The Prime Minister had denied reports of talks with the Taliban under questioning from David Cameron, the Tory leader, in Parliament.

Liam Fox, the shadow defence secretary said: "If this turns out to be untrue the Prime Minister will have some explaining to do to the British public."

Britain has said it would support efforts by the Afghan government to negotiate with tribal fighters now supporting the Taliban - but only if they embraced democracy.

Senior Government sources have claimed that the only negotiations with the Taliban were attempts by President Karzai to persuade them to change sides.