The Defence Secretary said during question time he expected "a significant reduction after around six months in the number of British troops committed to Kosovo" from a maximum of 13,000.
He acknowledged Britain's armed forces were "having to put up with very considerable strains" because of the demands being made of them. "At peak, we will be deploying up to 13,000 ground troops as part of K-For, with up to some 7,000 remaining ready in the UK for deployment in the region if needed."
Britain also had a "considerable commitment" to S-For inside Bosnia, he added. "In addition, some 4,500 personnel are serving with S-For in Bosnia. As of today, 900 Royal Navy personnel and 1,800 RAF personnel are also deployed in support of operations in the Balkans. The size of these operations will be kept under review."
Later, during a statement on last weekend's G8 Summit, Tony Blair hailed the withdrawal of Serb forces from Kosovo as "a huge achievement" for Nato.
To cheers in the Commons, the Prime Minister said: "The progress made in the few days since Milosevic finally caved in has been extraordinary, on the withdrawal of Serb forces, on the deployment of ours, on the role of the Russians, and now the agreement to demilitarise the KLA.
"This is a remarkable story. Britain and British forces can be very proud of their role in it."
William Hague welcomed the withdrawal of Serb forces and the demilitarisation of the KLA, adding the international community needed to demonstrate it was "doing its utmost to bring to justice" those responsible for war crimes committed in Kosovo.
Menzies Campbell, for the Liberal Democrats, asked for clarification of the humanitarian assistance to be offered to Serbia, saying many Britons would support it for a people "who through no fault of their own have found the infrastructure of their country substantially degraded and destroyed".
Mr Blair said the Serbian people must be aware of the mass graves and evidence of racial genocide seen by the world. "I think they do have some responsibility to make sure they send a clear message to their own government and their own regime."
But earlier Mr Robertson stressed that the international community would have to pay a "very substantial price" for the reconstruction of Kosovo. Much of the costs of the British forces' role would be met by the Treasury from reserves rather than from the defence budget.
Mr Robertson conceded that Nato miscalculated the size and firepower of the Serb forces stationed in Kosovo.