Tony Blair has expressed his sorrow at the "cowardly and disgraceful" terrorist bombing in Saudi Arabia.

The prime minister told MPs that the atrocity would "make the UK and our allies across the international community only more determined to track down terrorists and stamp out terrorism".

A silent House listened as he sent his "profound condolences" to the families of victims and the Saudi authorities.

The full extent of the casualties is still unknown, the prime minister said, but "tragically some of these victims may be American and indeed British".

With the bomb thought to be the work of Osama bin Laden's al Qaeda network, Blair said the actions were self-defeating.

"As ever with these terrorist atrocities, the victims will be primarily Muslims," he said.

One British man is thought to have been killed in the attack and five Britons - including a 15-year-old girl - are believed to have been injured.

Earlier, the Foreign Office said two Britons were unaccounted for and their relatives were being contacted.

The government has amended its travel advice to recommend against all but essential trips to the country.

Conservative leader Iain Duncan Smith echoed the prime minister's sentiments.

"May I join you unreservedly in your comments with regard to the atrocious bombing in Saudi Arabia," he said.

"I think the whole House will share your view on that without any question at all."

Blair also said he was "deeply concerned" about the cases of Britons in jail in Saudi Arabia for alleged bombings and promised to work "vigorously" for their release.