Prime Minister Mr Tony Blair addressed the British people last night in an appeal for national unity and resolve behind the country's forces at war in Iraq.
Mr Blair's pre-recorded broadcast confirmed the substantive engagement of British troops as the Allies' promised "shock and awe" assault on the Iraqi regime gathered pace, and as the British Defence Secretary, Mr Geoff Hoon, echoed President Bush's warning that the military campaign might take longer than forecast.
With the bid to topple Saddam Hussein begun in earnest, Queen Elizabeth also sent a moving message to British personnel entering combat. "May your mission be swift and decisive, your courage steady and true, and your conduct in the highest traditions of your service both in waging war and bringing peace," the monarch told them.
"My thoughts are with you all, and with your families and friends who wait at home for news and pray for your safe return."
Buckingham Palace said the queen also sent a message of support to Australian forces serving in the Gulf.
Mr Blair, who recorded his broadcast before flying to Brussels for the EU summit, had remained silent throughout the earlier part of the day.
Mr Blair confirmed that Britain was officially at war. He saw the surprise, intelligence-led attack on key elements of the Iraqi leadership early yesterday morning as "preliminary" to the planned major commitment of forces.
Mr Blair's official spokesman said this was why Mr Hoon had made the statement about the overnight action to MPs yesterday.
With today a non-sitting day, Mr Blair's first opportunity to report to the House of Commons on the gathering conflict will be on Monday.
The Prime Minister's spokesman also dismissed suggestions that the British had been "taken by surprise" by the opening of hostilities shortly after the expiry of the US deadline for Saddam to leave Iraq.
"We work together, interwoven, and there are discussions about targets and strategy and tactics going on all the time," he explained. "To suggest that in some way people are keeping back information is just to fundamentally misunderstand the process."
At breakfast time yesterday, Mr Hoon told the BBC that British personnel had been involved in the US-led strike at the Iraqi leadership, understood to be the result of intelligence obtained by the CIA.
"I was well aware it was going to happen (on Wednesday night)," he said. "I was involved in discussions about these operations well before they occurred. There is full transparency between the two militaries. We operate as a coalition, together.
"There is a full exchange of information, certainly up to the level of Defence Secretary in the US and Secretary of State in the UK. Obviously the Prime Minister is informed as and when necessary."
A senior Whitehall source last night agreed that Mr Hoon's assertion of British involvement in the first missile strikes on Baghdad confirmed British involvement in the prior decision-making process, since all military operations have to be approved by representatives of the government's law officers deployed to the theatre of operations.
British sources at central command in Qatar also confirmed that the UK command had "full knowledge" before the strike, and that Air Marshall Brian Burridge, commander of UK forces in the Gulf, had informed the Ministry of Defence and the Prime Minister about the fast-moving situation at around midnight UK-time.
In his Commons statement, Mr Hoon told MPs: "We certainly hope that offensive operations will be over quickly."
However, he also warned: "We should not underestimate the risks and difficulties we may face against a regime that is an embodiment of absolute ruthlessness, with utter disregard for human life."
The Foreign Secretary, Mr Jack Straw, also braced the British public for inevitable civilian deaths during the conflict, telling a news conference: "The number of Iraqi lives saved by this action will far exceed the number of people who sadly will be killed."
Mr Straw continued: "It's a terrible calculation to have to make, but it's one that you have to make if there is to be proper justification for military action."
The allied coalition, he said, had been left with "no option" but to use force because of Saddam's failure to disarm.
"He has rejected that opportunity by once again responding with his familiar pattern of deceit, concealment and delay," said Mr Straw, adding: "Every single thing we are doing is fully in compliance with (UN) Resolution 1441. What we are doing is putting that resolution into effect."