UK troops are likely to be involved for just a few months
Former defence chief of staff Lord Guthrie suggested that Britain might be taking on too much by deploying large numbers in the troubled country.
He added that he shared Lord Guthrie's concern that British military resources should not be overstretched.
General Lord Guthrie of Craigiebank, Field Marshal Lord Inge, Field Marshal Lord Bramall, and RAF Marshal Lord Craig of Radley all expressed concerns during a House of Lords debate on the deployment of peacekeepers.
Lord Guthrie, who stepped down as Britain's defence chief earlier this year, said: "We must avoid falling into the trap of becoming so mesmerised by Osama Bin Laden and al-Qaeda that other key parts of defence are neglected and underfunded and we are found unprepared when confronted by a new threat."
'Overcommitted'
He added: "For we live in dangerous times and we can be absolutely sure that new threats will appear."
Lord Inge said the armed forces were "already dangerously overcommitted", while Lord Bramall argued the deployment would overstretch British military resources.
Their comments follow a Commons statement by Prime Minister Tony Blair in which he said that an international military presence was essential to future stability in Afghanistan.
"The situation in Afghanistan remains fragile, the new political process remains in its infancy," he said.
"There is therefore an urgent need to ensure that, as the war is being won, we play our part in securing the peace."
Conservative leader Iain Duncan Smith said he had "deep misgivings" over the deployment of British troops in a peacekeeping role when special forces were still actively engaging remaining Taleban fighters.
But Mr Hoon told the BBC that British troops would have "robust rules of engagement" in role of providing "security assistance" to the fledgling Afghan government.