LONDON -- British Prime Minister Tony Blair came under fresh pressure on Monday to allow Parliament to have a say in whether or not he joins any U.S.-led military action against Iraq.

One of Blair's most vocal critics in his own Labour Party, veteran member of Parliament Tam Dalyell, told Blair that for MPs the issue of Iraq was the "most important decision of their political lives".

In an open letter to the prime minister, Dalyell -- one of Labour's most senior MPs -- said Blair had a moral obligation to recall Parliament from its summer break and debate the issue with his political colleagues.

His intervention followed a warning from former British defense chief Lord Bramall that Blair risked being dragged into a long, messy Middle East war if he joined the United States in a military assault to topple Iraqi leader Saddam Hussein.

It also coincided with the Royal Navy's decision to send its flagship aircraft carrier, Ark Royal, to the Mediterranean, prompting media speculation it was being positioned for a future attack on Iraq.

Both the Ministry of Defense and Blair's office immediately dismissed the speculation. A spokeswoman for Blair said it was "absolute rubbish". A Defence Ministry spokesman told Reuters: "This is a routine exercise, nothing out of the ordinary, and not in any way connected to anything to do with the Persian Gulf or speculation in the media about Iraq."

Both Blair and U.S. President George Bush have repeatedly said no decisions have yet been taken on whether to launch a military assault on Iraq and that a strike is not imminent.

But Blair is facing growing dissent from left-wingers in his Labour government and senior military figures who fear an attack may develop into a wider regional conflict.

Field Marshal Lord Bramall, Britain's former chief of defense staff, urged Blair to proceed very cautiously and warned that without United Nations backing, both Blair and Bush could be on shaky legal and moral ground.

"This is a potentially very dangerous situation, in which this country might be swept into a very, very messy and long-lasting Middle East war," he told the BBC on Sunday.

"You don't have licence to attack someone else's country just because you don't like the leadership. Nowadays, you are supposed to get UN backing for all this."

In his letter to Blair on Monday, Dalyell said: "In circumstances in which a distinguished chief of the general staff feels obliged to...warn of a very messy and long-lasting Middle East war...do you not have a moral obligation to ask for the recall of Parliament in early September?" But Blair's spokeswoman said the prime minister was not prepared to be "pinned down" to any specific form of consultation with his parliamentary colleagues.

"We are not at the point of decisions. There are many issues to consider before we are at the point of decision," she said.