Foreign Secretary William Hague: "We keep our travel advice under careful and constant review"
British nationals in Cairo, Alexandria and Suez are being told to leave if it is safe for them to do so, following days of violent protests across Egypt.
But despite upgrading its advice, the Foreign Office (FCO) is not currently organising a formal evacuation.
Foreign Secretary William Hague said he was "very concerned" by the large number of Britons trying to leave at Cairo airport.
Most of the 20,000 UK tourists in Egypt are in Red Sea resorts considered safe.
Mr Hague said that although flights were coming in and out of Cairo airport, there was a lack of staff there which meant it was not functioning properly.
He said the welfare of British nationals was his top priority and that he had sent extra consular staff to the airport.
The British Ambassador to Egypt, Dominic Asquith, said: "There are a lot of challenges at the airport in Cairo. That's why we've got the team up there trying to help. There are flights going in and out but it is not orderly."
Several flights to Britain have been cancelled, people are unable to use the internet to book, and schedules have been affected by the curfew imposed in the city.
The UK Foreign Office is warning against all non-essential travel to Cairo, Alexandria, Luxor and Suez, and say that anyone "without a pressing need" to be in those cities, excluding Luxor, should leave if it is safe for them to do so.
FCO minister Alistair Burt told the BBC: "We do want people to take the opportunity if they are able to leave... but as yet the situation has not reached the stage where we would necessarily be considering chartering planes and getting larger numbers out."
In line with that advice, the FCO says arrangements are being made for the spouses and children of British embassy staff in Cairo to leave the country on ordinary commercial flights.
Mr Hague also warned that Egypt could fall into the hands of extremists unless there were peaceful reforms.
He told the BBC: "It's to avert those risks and meet the legitimate grievances and aspirations of the Egyptian people that we are urging the Egyptian authorities... to create a more broadly-based government."
He said reforms should be "real and visible" and elections "free and fair".
The US embassy in Cairo is telling Americans to consider leaving the country as soon as possible and will begin evacuation flights on Monday.
In Cairo, demonstrators are back on the streets for a sixth day, demanding the resignation of President Hosni Mubarak.
The FCO said Britons should adhere to a nationwide curfew imposed between 1600 and 0800 local time, and stay indoors if they are in an area that has experienced demonstrations.
Rosemary Turberville, who is from London and staying in a suburb of Cairo, told the BBC: "The gunshots were evident last night... but in my particular area [now], it's pretty tranquil.
"I'm on the 10th floor. I can see people wandering about, some people with bags of shopping, so there may be a shop open down there, but I have plenty of supplies so I don't see the point in risking going outside."
Alex Belfield, a presenter with BBC Leeds who is on holiday in Sharm El Sheikh, said tourists at his hotel had been told they could not leave the building for at least 24 hours.
'Democratic legitimacy'"This whole thing seemed a million miles away yesterday, but last night... when we got back [from dinner] the whole atmosphere had changed.
"The barricades had gone up at the front of the hotel... and there are 14 security guards in total, making it very clear we were not able to leave."
But Sean Tipton, from travel body Abta, said the introduction of tight security measures was not being reported by other customers in Sharm El Sheikh and the resort was "almost totally unaffected".
Mr Tipton said UK tourists required to travel to Luxor, which is a popular start and end point for Nile cruises, were "being taken to the cruise very quickly and got out of the place very quickly".
Mr Burt also said the FCO had "absolutely no reports of anything untoward happening in Sharm El Sheikh or any of the other resorts", but some hotels appeared to be "taking their own precautions".
In other travel news:
- British Midland International (BMI) said it would operate flights between Heathrow and Cairo on Monday but would change the times to operate outside the curfew. Its flights to Cairo were cancelled on Sunday
- Tour operator Thomson has cancelled departures to Aswan - a city on the Nile in southern Egypt - and Luxor until further notice, but the company will send its planes out empty to bring home holidaymakers due to leave
- British Airways and Egyptair have altered their schedules
- Easyjet's services to Luxor are operating, but passengers have been told they can rebook for a later date
- Some travel companies have cancelled excursions from Red Sea resorts to Cairo and ancient Egyptian sites in Giza and Luxor
- The Independent's travel editor, Simon Calder, said package holidays from the UK to Red Sea resorts were going ahead as normal, and tour operators had no liability to anybody who decided not to go.
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