Defence Secretary Des Browne told MPs that it was "prudent" to "pause" the planned drawdown of British forces which had been expected to see troop numbers fall from their current level of around 4,000 to 2,500.

In a Commons statement, he said the decision had been taken on military advice in the light of the changing situation on the ground.

The announcement drew an angry response from the Tories who accused ministers of "playing party politics" with troop numbers.

The intention to cut British forces was first signalled by Gordon Brown during a surprise visit to Iraq last October.

The Prime Minister later announced the planned reduction to 2,500 from this spring in a Commons statement.

Mr Browne said that even before Iraqi Prime Minister Nouri al-Maliki launched his offensive against the militias last week, the "emerging military advice" was that it would not be possible to reduce numbers as quickly as had been planned.

"In the light of last week's events, however, it is prudent that we pause any further reductions while the current situation is unfolding," he said.

"It is absolutely right that military commanders review plans when conditions on the ground change."

Shadow defence secretary Liam Fox said the delay would have a "marked effect" on the existing "overstretch" of the armed forces.