The Foreign Office has advised against all non-essential travel to Saudi Arabia, Jordan, Bahrain and Qatar.
Holidays to Petra in Jordan had been popular
It also urged Britons in those countries to be vigilant, and to consider
whether their continued presence was essential.
BAE Systems employee Robert Dent, 37, was shot dead in Saudi Capital Riyadh on Thursday, by a gunman who
pulled up alongside him as he waited at traffic lights.
Mr Dent's death came less than a fortnight after another BAE employee escaped
with minor injuries after also being shot at while driving his car in Riyadh.
Jordan has been a recent popular holiday destination for Britons, but the Foreign Office website made clear holiday travel was included in the warning.
The reasons given for all four countries were "increasing regional tension and the risk of terrorist action".
Non-essential UK embassy staff, and relatives of embassy staff, were being allowed to leave.
On Thursday, British citizens were given similar advice for Kuwait, Israel and the Palestinian Territories.
And they were advised to leave Iraq immediately because of the stand-off with Saddam Hussein.
Middle-East countries which have not featured in recent warnings include Turkey, Iran, Oman, Lebanon, Syria, Egypt, and the UAE.
The Foreign Office had already been advising against travel to Yemen.
However, the Foreign Office says developments on Iraq could affect the advice for all countries in the region, so its website should be checked daily.
The latest advice has fuelled speculation about possible war.
Thousands of American and British troops are in Kuwait, which would be likely to form a key base for an invasion of Iraq.
Some politicians and Britons living abroad have questioned the recent spate of travel advice.
Graham Hawkins, from the British School of Kuwait, on Thursday told the BBC many of the 3,500-4,000 Britons in the country would stay despite the new advice.
"There is a feeling that something will inevitably happen in the not too distant future but also... that it is unlikely directly to affect us here," said Mr Hawkins.