UK commanders were keen to redeploy Army troops leaving Iraq to Afghanistan because of fears they would be cut if they were not used, it is claimed.
Former envoy to Kabul Sir Sherard Cowper-Coles told MPs he was told by ex-Army head Sir Richard Dannatt the conflict gave them a "raison d'etre".
But Sir Richard has strongly denied the claims, calling them "somewhere between misjudged and mischievous".
The MoD said UK forces were deployed for "one very clear reason".
A spokesman said UK national security was the only motivation for sending troops to the country.
"Alongside international forces from 48 other nations we are there to prevent al-Qaeda from again using Afghan territory to plot and launch terror attacks," he said.
Sir Richard said he "failed to recognise" comments attributed to him by Sir Sherard, who made the claims in written evidence to the Commons Foreign Affairs Committee.
Sir Sherard said he had been told by Sir Richard that if he did not re-deploy battlegroups coming free from Iraq, he would lose them in a future defence review.
In the written memorandum to MPs, the former envoy said the Afghan campaign had seen "unprecedented" resources diverted to the Army, and that most soldiers appeared to be "enjoying" it.
Sir Sherard said British commanders also saw the mission in Afghanistan as an opportunity to redeem their reputation in the eyes of the Americans after criticism of their performance in Basra.
"The war in Afghanistan has given the British Army a raison d'etre it has lacked for many years, and new resources on an unprecedented scale," he said.
'Use them or lose them'
He went on to say that Sir Richard, who was then Chief of the General Staff, told him - in the summer of 2007 - "if he didn't use in Afghanistan the battle groups then starting to come free from Iraq, he would lose them in a future defence review".
He said he was told it was a case of "use them, or lose them".
Sir Sherard added: "In my view, the Army's 'strategy' in Helmand was driven at least as much by the level of resources available to the British army as by an objective assessment of the needs of a proper counter-insurgency campaign in the province."
Sir Sherard has been a high-profile critic of the military strategy in Afghanistan since he stood down from his diplomatic role in Kabul last year, says BBC defence correspondent Jonathan Beale.
Sir Richard described Sir Sherard's claims as "distressing".
"I have great respect for Sherard Cowper-Coles as a diplomat but believe that many of his comments with regard to the military are somewhere between mis-judged and mischievous - they are largely based on snapshots and passing conversations," he said.
"He has strayed out of his lane in a most unfortunate manner - particularly on issues such as unit tour lengths and Army organisation. I would not dream of telling him how to organise an Embassy.
"His most distressing comments relate to the suggestion that the Army wanted to become increasingly involved in Afghanistan. Any soldier who has seen any action and the casualties that result does not go looking for more."
He said the decision to go to Afghanistan was taken by the government in "some haste" in 2004.
"From that moment on we were committed to two campaigns while only resourced for one. From 2006 to 2008 this was a major juggling act to balance.
"I fail to recognise the comment attributed to me about using battlegroups or losing them. The facts are that once we had withdrawn from Iraq more battlegroups were available and were desperately needed in Afghanistan to provide more boots on the ground, an essential prerequisite to success in a counter-insurgency campaign.
"I was never worried about the Army being thought to be big; my worry was that it was too small for all that it was being asked to do."
Sir Sherard stepped down from his post as the UK's most senior diplomat in Afghanistan in September. He had been on extended leave since June and was reported as saying he was going because his job was done.
The BBC's Orla Guerin said last summer that some believed his departure was down to his straight-talking and he had undermined himself by getting involved in too many disputes.
The number of British military deaths in operations in Afghanistan since 2001 stands at 349, after a soldier from Argyll and Sutherland Highlanders, 5th Battalion The Royal Regiment of Scotland died on 1 January 2011, in Helmand.
There are about 9,500 UK troops in Afghanistan, with the majority deployed in the south of the country.