Britons have been warned to steer clear of Egypt's main cities as violent demonstrations threaten to overthrow the government.

With President Hosni Mubarak facing the biggest challenge of his three decade rule, the Foreign Office cautioned against "all but essential" travel to Cairo, Alexandria, Luxor and Suez.

Tourist sites have also been shut down, while tanks were said to be stationed outside of Western embassies.

Foreign Secretary William Hague said: "In light of the ongoing demonstrations in Egypt we have carefully reviewed our advice and now advise against all but essential travel to Cairo, Alexandria, Luxor and Suez. This does not affect transits through Cairo airport for onward travel to other destinations, and it does not cover Egypt's Red Sea resorts."

The Foreign Office confirmed the advice extended to include Giza, the famous pyramid site near Cairo. Meanwhile, tour operators including Thomson and First Choice have cancelled excursions to Cairo until further notice.

A spokeswoman for travel association Abta said: "The vast majority of British holidaymakers in Egypt are based in resorts in the Red Sea area, for example Sharm el Sheikh and Hurghada. This is a very considerable distance from the affected areas and these resorts remain unaffected. Any tourists based in Cairo, Alexandria, Luxor or Suez should liaise with their tour operator or airline regarding their holiday arrangements."

Nobel Peace Prize winner Mohamed ElBaradei, a leading opponent of Mr Mubarak, was put under house arrest almost immediately after returning to the country to join the dissidents.

The president attempted to regain the initiative in the volatile situation on Friday night by making his first public address. "I have asked the government to present its resignation today," he said on state TV. And he insisted he understood the protesters' grievances but that a thin line divided liberty from chaos, and he would not allow Egypt to be destabilised.

Meanwhile, around 50 Britons were forced to spend the night in hotels after an Egypt-bound passenger plane was forced to make an unscheduled landing when a note containing a bomb threat was apparently found on board. The EgyptAir jet, which had departed from London's Heathrow Airport, was carrying 251 passengers when it landed at Athens International Airport, officials said.

A BA spokeswoman confirmed later that a chartered aircraft was on the way to Cairo to bring back as many passengers as possible to Gatwick.