The BBC's Jon Kay: "Tourists are being warned not to go to northern Turkey"

The earthquake, measuring 6.8 to 7.0 on the Richter scale, killed 3,000 people across the northwest of the country, with its epicentre near the city of Izmit, about 65km east of Istanbul.

Istanbul was badly hit by the quake, with hundreds killed and thousands injured in the city alone.

The UK Foreign Office says two British nationals were injured, but their condition is not life threatening.

The tremor did not affect any of the south-west coastal resorts which are most popular with British holidaymakers, such as Bodrum, Fethiye and Marmaris.

Tour operators say holidays to these regions are operating normally, and holidaymakers would not be able to pull out without penalties.

The Foreign Office has set up a telephone line, on 0171 839 1010, for latest updates on the situation across the whole country. Travellers with specific inquiries should contact 0171 839 5656.

Historic sites escape

International Development minister George Foulkes announced an immediate grant of £500,000 to boost the Red Crescent's appeal for international assistance.

It is also giving £32,000 to the United Nations Office for the Co-ordination of Humanitarian Affairs to help strengthen local efforts to co-ordinate international response to the emergency.

"Tonight we are flying out a team of experts with specialist rescue equipment from fire brigades around the UK, as well as the International Rescue Corps, all skilled in search and rescue techniques," he said.

There have been no reports of any British casualties in Istanbul, but communications have been affected and operators were having difficulty assessing the damage.

The Turkish Tourist Board said that although emergency services were on full alert, the historic tourist areas of Istanbul were unaffected and the city's airport was open.

There was no visible damage to the Blue Mosque

The Association of British Tour Operators (Abta) said that anyone due to visit Istanbul in the next few days should reconsider, but that after that more information would become available on which areas were safe to visit.

First Choice Holidays has two people in Istanbul, and Turkey-specialist company Tapestry has four. Both companies said that they were having difficulties getting through to Istanbul to find out if their people were all right.

A First Choice spokeswoman said the company was monitoring the situation on a day-to-day basis, but that anyone due to fly to Istanbul in the next seven days had been offered a full refund.

However, there has been no visible damage to historical sites such as the Blue Mosque, Saint Sophia or Topkapi Palace, which are visited by thousands of tourists each summer.

And British Airways, which runs two scheduled flights a day from Heathrow to Istanbul, said flights were running normally.

'Tragic loss'

Thames Water said 21 of its 82 staff in Izmit remained unaccounted for. It has been confirmed that one Turkish employee had lost his life.

The company's 12 British staff had been accounted for, it said.

Chief executive Bill Alexander said: "This is a tragic loss for the family of our employee and our thoughts are with them at this time.