In a statement in the House of Commons, Mr Blair said that British troops stationed around Kosovo would increase from 4,500 to 6,300.

But the prime minister stressed the build-up did not mean Nato was planning for a ground invasion.

"Let me make clear, for the avoidance of doubt, they are being sent so that the UK can be in a position to play our proper role in the international effort to ensure the refugees are able to return to Kosovo in safety."

Tony Blair: "President Milosevic will be held responsible"

He told the Commons that the "murder, rape and terror" Serbian President Slobodan Milosevic had perpetrated in Kosovo had "justified Nato's military action".

The prime minister said that ethnic cleansing in Kosovo "must be defeated and must be seen to be defeated".

Mr Blair also detailed the terms under which the conflict could be settled.

He called for a withdrawal of Serbian forces from Kosovo and the acceptance by President Milosevic of an international peace keeping force on the ground, to ensure the safe return of refugees.

'A million have fled'

The BBC's Robin Oakley reports on the debates in Parliament

The prime minister told MPs that over half a million Kosovan Albanians had left Kosovo. A similar number had been forced from their homes and were still inside the province.

Mr Blair said that President Milosevic would be held responsible for the welfare of the refugees.

He said: "When we go into Kosovo finally, he will be held responsible for what we find."

Other Nato members are expected to announce similar troop reinforcements.

The British army has a total of 110,000 troops. The additional 1,800 being sent to the Balkans join the 4,500 already in the region.

Tony Blair: We are sending substantial reinforcements

The forces are stationed in the former Yugoslav republic of Macedonia and are equipped with 14 Challenger main battle tanks, 40 Warrior armoured fighting vehicles and six AS 90 self-propelled guns.

'Permissive environment' questioned

William Hague: We continue to support the airstrikes

The leader of the opposition, William Hague, backed the prime minister's announcement, saying Nato's involvement in Kosovo "is vital for Nato and the stability of the Balkans but, above all, it is vital for the people of Kosovo".

Alan Clark voiced concerns over the raids

But he added that for it to be successful there had to be a "clear and consistent" strategy.

Liberal Democrat Leader Paddy Ashdown asked the prime minister whether the "permissive environment" required by Nato before ground forces could be committed to Kosovo meant when the "risks are acceptable" or when President Milosevic permitted it.

Paddy Ashdown: There will be setbacks and casualties

Mr Ashdown added he "cannot have a veto as to whether or not we should send ground troops in" as that would be "totally unacceptable".

But Nato's bombing of Serbia was criticised by former Tory minister Alan Clark.

He accused the prime minister of "associating this House and this country with the sustained bombardment of a brave and Christian people who have never injured or even threatened a British citizen".

'Planning for success'

Jon Sopel reports: "There could be other similarly linked announcements"

Speaking before the announcement, at a Ministry of Defence briefing, Foreign Secretary Robin Cook said: ''These extra resources that we are sending to the theatre also help us plan for success.

"When the time comes for the refugees to return we will be ready to give them the international protection they will need," he said.

Sir Charles Guthrie, Chief of Defence Staff, added that although the Nato force (Kfor) would pack a "significant military punch" it was not intended as a fighting or invasion force.

Mr Cook revealed that three times as many Nato planes had taken part in bombing raids overnight than in previous attacks with RAF Harriers and Tornados fully involved.