The countdown towards Britain’s exit from the war in Afghanistan could start as early as next year, Gordon Brown told a City audience last night.

He announced that a summit of Nato allies in London in January would set a process for transferring to full Afghan control district by district, and “if at all possible set a timetable for transfer starting in 2010”.

Although No 10 insisted that it would not be an “exit summit”, the in- clusion of the 2010 date was clearly designed to assure an increasingly sceptical British public that the Afghan mission was not indefinite. Next year is also election year in Britain.

Mr Brown told the Lord Mayor’s Banquet at Guildhall, that he had offered London as a venue so that the international community could agree plans “for the support we will provide to Afghanistan during the next phase. I want that conference to chart a comprehensive political framework within which the military strategy can be accomplished,” he said.

The Prime Minister is clearly hoping to persuade Nato allies to send more troops and military resources in the short term, to ensure that the mission can be achieved quickly. British troops would be unlikely to start leaving before 2011, to ensure that the district handovers work satisfactorily.

In an attempt to appear upbeat about the war, Mr Brown added that the mission was inflicting heavy blows on the al-Qaeda leadership.

He said that that since January last year, seven of the top dozen figures in the network had been killed, “depleting its reserve of experienced leaders and sapping its morale”.

The security services were now reporting to him that there was an opportunity to inflict “significant and long-lasting damage to al-Qaeda”, he said. “We understand the reality of the danger and the nature of the consequences if we do not succeed; we will never forget the fatal al-Qaeda-led attacks in London on July 7, 2005.”

Mr Brown’s speech was the latest in a series trying to meet criticism that the Government is not doing enough to explain the reasons for sending troops to Afghanistan, and to give people hope there is an end-game.

Mr Brown said the action in Afghanistan and Pakistan was necessary because al-Qaeda remained the biggest threat to Britain’s security. “More has been planned and enacted with greater success in this one year to disable al-Qaeda than in any year since the original invasion in 2001.”

But he said that Britain was there “because we judge that if the Taleban regained power, al-Qaeda and other terrorist groups would once more have an environment in which they could operate”.

The terrorist network had an extensive recruitment operation across Africa, the Middle East, Western Europe and the UK; and there were several hundred foreign fighters based in Pakistan travelling to training camps to learn bombmaking and weapons skills.

Since 2001 nearly 200 people had been convicted of terrorist or terrorist-related offences in Britain. “And to those who say this threat is not real, I ask them to consider that almost half of those convicted pleaded guilty.”

Turning to Iran, Mr Brown said that the West’s offer of engagement and negotiation had been comprehensively rejected. It was now right and necessary for the world to apply concerted pressure to the Iranian regime. President Obama had set an end of year deadline for Iran to react. “If Iran does not reconsider then the UN, the EU and individual countries must impose tougher sanctions,” Mr Brown said.

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