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One of the bombs went off at the British consulate, the other at the London-based bank, HSBC.
The top British diplomat in the city, Consul-General Roger Short, was among at least 14 people killed at the consulate.
The British Foreign Office also confirmed that another Briton, diplomatic staff member Lisa Hallworth, was among those who died.
The Turkish authorities say the attacks were carried out by suicide bombers, reportedly with links to al-Qaeda.
A man has called the semi-official Anatolia news agency to claim that al-Qaeda and the Turkish Islamic militant group IBDA-C had jointly carried out the attacks.
The bomb at the consulate was so big that buildings hundreds of metres away had their windows blown out.
The multi-storey building which housed the bank headquarters overlooked a crowded shopping centre, filled with people at the time the bomb went off.
The city was thrown into chaos by the blasts. Traffic blocked ambulances as they tried to reach the wounded, and much of the city's phone network was cut off.
The British Prime Minister, Tony Blair, condemned the attacks, and said there could be "no holding back" in confronting the "menace" of global terrorism.
The Foreign Minister, Jack Straw, is already on his way to Istanbul.
The United States has closed its consulate and warned its citizens to stay away from the area where the bombings took place.
US President George Bush said the bombers had shown "utter contempt" for human life.
The Turkish Prime Minister, Tayyip Erdogan, said he would track down those responsible.
"Turkey will be like a fist [against the culprits]... The best response for us is to stay calm in the face of terrorism," he told journalists.
The bombs come less than a week after suicide bomb attacks against two synagogues in Istanbul in which 25 people died.
A group claiming to be linked to al-Qaeda has also claimed responsibility for those attacks, and warned that new attacks against the US and its allies are being planned.
The death toll was finally put at 32, among them three Britons. The third was another consulate employee, Nanette Kurma.
An investigation concluded that a local paramilitary group called Hezbollah was responsible for planning the bombings, with evidence of al-Qaeda involvement at a high level.
Within days police had begun arresting suspects.
In February 2004, a total of 69 people were charged in connection with both the synagogue bombings and the consulate and bank bombings.
Five of them were described as "leaders of the al-Qaeda cell in Turkey".
The indictment also named nine key suspects who were still at large, probably outside Turkey.
They included the alleged head of the Turkish al-Qaeda cell, Habib Akdas, accused of masterminding the attacks.
In September 2004, Akdas was reported killed in Iraq during a US raid.
The original trial began in May 2004, but was postponed after it was found that the State Security Court in Istanbul had been abolished by the government as part of pro-EU reforms.
Hearings began again in September 2004.
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