By putting some of Britain's most highly trained combat units on stand- by to join 45,000 extra troops in Nato's K-For ground force, Tony Blair sent out the clearest signal so far that Britain and Nato are prepared to fight their way into Kosovo to return the refugees to their homes.
It is also an admission that the air campaign alone is not enough to force Slobodan Milosevic into submission.
The announcement came amid reports that the International War Crimes Tribunal for former Yugoslavia, based in The Hague, is about to indict Mr Milosevic formally as a war criminal, a development that will effectively close the door to a negotiated diplomatic end to the conflict.
Britain's offer to send 12,000 troops in addition to the 5,400 already deployed in Macedonia is intended to take pressure off President Bill Clinton in the face of failing public support in the United States for American troops to be used on the ground in Kosovo.
The Secretary of State for Defence, George Robertson, denied assembling an invasion force, saying that the extra troops would be needed to rebuild Kosovo's infrastructure after the war. But military experts were quick to note that Britain is sending a hard-hitting force more suited to fighting than to rebuilding houses.
Three infantry battalion groups were put on reduced notice to move - the first battalion of the Royal Irish Rangers, the first battalion of the Parachute Regiment and the first battalion of the Royal Gurkha Rifles. In addition, 3 Commando Brigade was put on standby with the Amphibious Ready Group, including HMS Ocean, Fearless and auxiliary vessels.
Mr Robertson described it as a "major and momentous undertaking". Paddy Ashdown, the Liberal Democrat leader, who has been calling for ground forces to be used, said the troops were "frontline arms", and the Prime Minister said it was a "significant and right move". Mr Blair appeared to accept that he was moving from peace-keeping to peace making in Kosovo.
"We shall make sure there is sufficient force so that the refugees can be allowed back home properly," he said. "It is important that we make sure ... that we have sufficient ground forces to do the job."
Nato decided on Tuesday that the planned total of 28,000 for K-For was not enough. The number deployed in the region could rise to between 55,000 and 60,000. However, British officials said yesterday that at least 200,000 troops would be required if Serbian opposition was encountered.
Mr Blair spoke to members of the Parliamentary Labour Party at a private meeting about the need to force the Serbian troops into retreat from Kosovo, and assured the MPs that Nato would win.
Mr Robertson's Commons statement came amid mounting evidence that Nato could secure widespread support for a ground force to enter Kosovo in a "non-permissive environment" if the air campaign and diplomacy fail to dislodge Mr Milosovic's men.
As the Foreign Secretary, Robin Cook, toured three European capitals yesterday it emerged that the Germans would not block the entry of Nato forces into Kosovo ahead of a peace agreement, in spite of Chancellor Schroder's earlier statement that a land campaign was "unthinkable".
British officials believe Italy might - if it came to it - take the same line. Lamberto Dini, the Italian Foreign Minister, stressed after his meeting with Mr Cook the importance of the diplomatic process, but he agreed that it had to be backed by firm military action. He did not repeat earlier Italian proposals for a "bombing pause".
Nato expects Paris to increase its forces in line with the overall rise in Nato numbers, implying a doubling of the existing French K-For troop numbers from 4,500.
American forces are to be increased from 4,000 to 7,000, and increases are also expected from Italy, Germany and the Netherlands.