Britain's military force in the Gulf is "rapidly building", Geoff Hoon has told MPs.

The defence secretary said that allied forces were "continuing to enforce the no-fly zones" but denied reports that they were now targeting key Iraqi military installations.

He reported that UK forces had been engaged in "more frequent patrols" and continued to act "in self defence and in accordance with international law".

They were "presenting a significant and credible threat of force," he argued.

"Detailed planning will continue to evolve but we currently estimate a total deployment of around 45,000 armed forces personnel. The United Kingdom presence in theatre is rapidly building to form a highly capable force," said Hoon.

Around 30,000 personnel were now in the region "making a significant contribution to the flexible and balanced range of coalition capabilities", MPs were told.

He also informed the Commons that the government had agreed to the deployment of 14 US Air Force B52 bombers to RAF Fairford in Gloucestershire.

Despite the military build-up, the defence secretary continued to insist that no decision to go to war had been made by the government.

"The aircraft began to arrive at the base today. This is part of our continuing contingency preparations," he said.

"No decision to commence military action has been made."

The shadow defence secretary, who has just returned from Kuwait, said the UK was now "on the cusp of great events".

Bernard Jenkin called on Hoon to be more "honest and open about what is really going on".

"The tactics now reflect the government's decision to help clear the way for the invasion of Iraq which itself requires the protection of British and American ground forces now massing to cross the Iraqi border," he said.

Downing Street had earlier denied that there had been a change in policy on the no-fly zones.

"The policy in the no fly zone continues is as it was. They patrol the no fly zones as they did before. They respond to the threats they encounter.''

But former Labour minister Doug Henderson accused ministers of presiding over a "slide into war".

"The Pentagon has acknowledged that the previous policy of attacking Iraqi military capacity that threatened our planes has now been changed to attacking any Iraqi military capacity," he said.

"If Downing Street believe that there is a different interpretation, then it is high time that they got their act together.

"It looks like Downing Street are riding on the coat-tails of the Americans, but don't understand what is actually happening."

Liberal Democrats argued that the defence secretary's denial "raises suspicions".

"When senior officials in the Pentagon brief that a more aggressive approach is being taken in the no-fly zones, hitting targets that threaten ground forces not air forces, a flat denial from the UK defence secretary raises suspicions," said Paul Keetch, the party's defence spokesman.

"The no-fly zones have no official sanction from the UN and it is imperative that they are not abused, especially when the United Nations Inspectors are hard at work in Iraq."