BRITISH interests around the world feared they were in the terrorists' sights last night after two massive bombs killed at least 27 people in Istanbul.

The explosions wrecked the Turkish headquarters of the London-based HSBC bank and the British consulate.

A number of Britons were among the dead, including Roger Short, the Consul General, and his personal assistant, 38-year-old Lisa Hallworth.

The blasts - which also left more than 450 injured - came five days after the weekend synagogue bombings in the city killed 23.

The latest attacks appeared designed to coincide with the state visit of US President George Bush to Britain.

Prime Minister Tony Blair immediately vowed that Britain would not "flinch" in the face of terrorism.

The Queen sent a message saying that she was deeply shocked at the attacks and expressed her sympathy to "all caught up in these evil acts of terrorism".

And the Archbishop of Canterbury, Dr Rowan Williams, who visited Istanbul and met 59-year-old Mr Short earlier this week, spoke of his "shock and grief".

He said: "These acts of violence achieve nothing but to hurt a whole community of all faiths and none."

Foreign Secretary Jack Straw, who flew to Istanbul last night, said the attacks had every hallmark of "al Qaida and its associates".

At least 15 of the victims were reported killed in the blast outside the consulate. Mr Straw said three or four British employees from the consulate had not responded to a roll call.

Scotland Yard announced it was sending 16 Anti-Terrorism Branch officers to assist the investigation.

The Foreign Office also despatched a rapid deployment team of officials to Istanbul to assist people caught up in the blast.

At a joint press conference at the Foreign Office, Mr Blair and Mr Bush denounced the killings and pledged that the attacks would not undermine their commitment to the reconstruction of Iraq.

"In the face of this terrorism there must be no holding back, no compromise, no hesitation in confronting this menace, in attacking it wherever and whenever we can and in defeating it utterly," the Prime Minister said. "I can assure you of one thing: that when something like this happens today, our response is not to flinch or give way or concede one inch.

"We stand absolutely firm until this job is done, done in Iraq, done elsewhere in the world."

Mr Bush said that despite the bombing, "good progress" was being made in dismantling the al Qaida network and that efforts would continue to "chase down" the terrorists.

"The terrorists hope to intimidate, they hope to demoralise. They are not going to succeed," he said.

Stephen Green, HSBC's group chief executive, said a number of the bank staff had been injured, and some may have died. However he said they had no intention of pulling out of Turkey.

"HSBC remains totally committed to its business in Turkey," he said.

An unidentified caller to a local news agency said that al Qaida and a militant group called the Islamic Great Eastern Raiders' Front, or IBDA-C, had jointly claimed responsibility for the attacks.

Turkish authorities blamed the same groups for the synagogue attacks.

The Foreign Office revised its travel advice for Turkey, warning against all non-essential travel to Istanbul.