The United States should send up to 45,000 extra troops to Afghanistan, a senior adviser to the American commander in Kabul has told The Times.

Anthony Cordesman, an influential American academic who is a member of a team that has been advising General Stanley McChrystal, now in charge of Nato forces in Afghanistan, also said that to deal with the threat from the Taleban the size of the Afghan National Army might have to increase to 240,000.

If Mr Cordesman’s recommendation reflects the view of General McChrystal, who recently presented the findings of a 60-day review of Afghanistan strategy to Washington, it would mean sending another nine combat brigades, comprising 45,000 American troops, in addition to the 21,000 already approved by President Obama. This would bring the total American military presence in Afghanistan to about 100,000, considerably closer to the force that was deployed for the counter-insurgency campaign in Iraq.

If General McChrystal believes that America should send nine more brigades — Mr Cordesman suggested it should be between three and nine brigades — there is bound to be pressure on Britain to send reinforcements as well. The British strength now is 9,000.

Writing in The Times, Mr Cordesman, of the Centre for Strategic and International Studies in Washington, said: “The insurgents may have lost virtually every tactical clash [against Nato troops], but they have expanded their areas of influence from a presence in some 30 of Afghanistan’s 364 districts in 2003 to one in some 160 districts by the end of 2008, while insurgent attacks increased by 60 per cent during October 2008 to April 2009 alone.

“Nato must change its strategy and tactics after years in which member countries, particularly the United States, failed to react to the seriousness of the emerging insurgency,” he added.

The US reinforcements already approved by Mr Obama include 8,000 Marines of the 2nd Marine Expeditionary Brigade who have arrived in Helmand province, replacing the British troops in the south of the province, and 4,000 US Army soldiers from the 5th Stryker Brigade, who are also arriving in the region.

Mr Cordesman appeared to confirm the strategy expected to be outlined by General McChrystal relating to the Afghan National Army. He says that the existing plan to increase numbers to 134,000 soldiers is inadequate. He says that it should be doubled to 240,000 by 2014, and the Afghan National Police should rise from 82,000 to 160,000.

To reach such levels, however, Nato would need to contribute thousands more troops to train the Afghans.

On Saturday in The Times, General Sir David Richards, who becomes Chief of the General Staff — the head of the British Army — on August 28, said he thought that Britain’s commitment to Afghanistan could last between 30 and 40 years, although he envisaged that troops would have to stay only for the medium term. He is expected to repeat the call made by General Sir Richard Dannatt, whom he is succeeding, for more British troops for Helmand.

Tonight a former head of the British Army said it would not be possible for Britain to meet its commitment to support Afghanistan for decades if ministers approved a proposal to cut three infantry battalions.

As part of a current internal Ministry of Defence review, a reduction in the size of the infantry, from 36 to 33 battalions, has not yet been ruled out, because of the short-term savings that would ensue from cutting back on manpower — a total of £60 million a year for the loss of the three battalions.

However, General Sir Roger Wheeler, Chief of the General Staff from 1997 to 2000, said that it was illogical to reduce the size of the infantry at a time when there were so many casualties in Afghanistan and when it was now accepted that the mission in Helmand province was going to continue for decades. “And the MoD would be saving peanuts,” he said.

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