Britons around the world must be vigilant for terror attacks as war with Iraq begins, the Foreign Office said.
The warning came shortly before the first US strikes on Iraq.
US and UK forces are already deployed in battle positions in the Gulf.
British and US warplanes took to the skies on Wednesday attacking Iraqi artillery positions in the southern 'no-fly' zone.
The Ministry of Defence said the attack was "standard no-fly zone activity" targeting systems which are a threat to US-led forces.
The Foreign Office has said that there is an "especially high risk" of indiscriminate terrorist attacks in public places on UK citizens.
A spokesman said: "You should be vigilant, take sensible precautions, be aware of local
sensibilities, monitor the media, and check our travel advice for the country
you live in or plan to visit."
The Home Office also advised UK citizens to keep tinned food, bottled water and a battery-powered torch in their houses in case of a major terror attack in Britain.
As UK forces awaited the order to go to war, the White House denied reports that US-led forces had moved into the demilitarised zone on the Kuwait-Iraq border.
A British army spokesman told the Reuters news agency only that soldiers had taken up "forward battle positions".
Defence Secretary Geoff Hoon is expected to announce on Thursday that the long-term partners of any British service personnel killed in the Iraq war will qualify for pension pay-outs.
That would apply to unmarried couples and each case would be examined on its merits.
On Wednesday the prime minister held a 20-minute telephone conversation with President George W Bush.
He also met with key Cabinet colleagues at 10 Downing Street ahead of the expected invasion of Iraq and .
We will do everything we can to minimise civilian casualties
and indeed maximise the possibilities of a swift and successful conclusion to any conflict
They included Home Secretary David Blunkett, Transport Secretary Alistair Darling, Deputy Prime Minister John Prescott and Foreign Secretary Jack Straw.
At prime minister's questions in the Commons, meanwhile, Tony Blair pledged to MPs that the UK would do everything in its power to ensure war against Iraq is swift and minimises civilian casualties.
His comments came the day he overcame a huge rebellion to gain Parliament's backing for war.
In the Commons, he said: "If there is a conflict and Saddam Hussein is removed, then the future for the Iraqi people will be brighter and better as a result."
Mr Blair voiced his admiration for the "dedication and commitment" of British troops in the Middle East.
"We will do everything we can to minimise civilian casualties and indeed maximise the possibilities of a swift and successful conclusion to any conflict," said Mr Blair.
Mr Blair said a post-war administration would be endorsed by the UN, keep the country unified, reflect its diversity and respect fundamental human rights and freedoms.
Responding to questions about whether Saddam could be tried for war crimes, Mr Blair warned senior Iraq officials that they would be "held accountable for what they have done".
Removing Saddam Hussein would be a war aim if that was what was needed to disarm Iraq, he said.
President Bush gave Saddam Hussein 48 hours to leave Iraq from 0100 GMT on Tuesday.
The Iraqi leader flatly rejected the ultimatum almost as soon as it had been made.
US officials have suggested Iraqi field commanders have been given the authority to use chemical weapons without seeking further orders from Baghdad.
US-led troops have now been told to begin taking their anti-nerve agent tablets.
Despite signs that recent opinion polls suggest the scale of public opposition to military action might be softening, Mr Blair still faces considerable opposition to war.