The BBC's James Robbins: A major commitment of troops

Foreign Secretary Robin Cook is to announce on Friday that the UK will put up to 8,000 troops on permanent stand-by to take part in UN peace missions across the globe.

Civilian police officers will also be made available to enter world trouble-spots.

But both the Conservatives and the Liberal Democrats say the plan could create problems of over-stretch for the army.

Under the plan UK troops will be made ready to join any UN-commanded force at only a few days' notice, Mr Cook told UN secretary general Kofi Annan.

The force will be "brigade strength" - between 6,000 and 8,000 soldiers.

It is expected to be made up of elite troops and specialists such as engineers and communication experts.

Robin Cook: Meeting with Kofi Annan

The force - the first of its kind from a major military power - is designed to circumvent the problem of getting a workable peacekeeping force onto the ground in time to nip potentially explosive situations in the bud.

Speaking before the announcement, Mr Cook said: "The problem for the UN is that every time there is a fire they have had to get down the blueprint and start to build a fire engine.

Agreement signed

"At least now they will have the components for the fire engine.

"We will be giving the UN an undertaking that when called upon we are prepared to commit [troops] for an operation.

"We will earmark some so they are ready to be used when the time comes."

France has also agreed to allocate troops to the force.

Kofi Annan: The stand-by force will speed up peacekeeping operations

Under the agreement, signed in New York, the troops would move in after the UN Security Council had approved action.

Speaking at a joint press conference with Mr Annan, Mr Cook said the UK would retain a veto over the use of its troops.

"It will always remain our decision to respond to the decision [of the UN] but I do not think we have let you down yet, Kofi."

The UN secretary general said he was grateful for the commitment and added that the initiative will help solve one of peacekeeping's toughest problems - the swift deployment of troops.

Menzies Campbell: Labour needs to spend more on the services

But shadow defence secretary Iain Duncan Smith warned that the UK's forces were already under pressure and committed to operations in Kosovo, Bosnia and Northern Ireland.

He said: "In the midst of all this, the government announce, out of the blue, that 8,000 men are to be put on permanent standby for the UN, with no explanation as to how this commitment is to be met in terms of manpower or cost, or indeed how the British armed forces can maintain their basic ability to meet British defence needs."

For the Liberal Democrats, foreign affairs and defence spokesman Menzies Campbell welcomed the move but said more troops would be needed.