The prime minister said it was reasonably clear the blasts were a terrorist attack designed to coincide with the G8 summit in Gleneagles.

He said he was flying back to London to hear reports from police and emergency services face-to-face.

But the G8 summit would continue in his absence, he said in a TV address.

Mr Blair said it was "reasonably clear" terrorists were behind the blasts.

"It is important that those engaged in terrorism realise that our determination to defend our values and our way of life is greater than their determination to cause death and destruction to innocent people in a desire to impose extremism on the world," he said.

"Whatever they do, it is our determination that they will never succeed in destroying what we hold dear in this country and in other civilised nations throughout the world."

Mr Blair said all the G8 leaders wanted the meeting to continue in his absence so "that we should continue to discuss the issues that we are discussing and reach the conclusions that we were going to reach".

"Each of the countries around that table have some experience of the effects of terrorism and all the leaders ... share our complete resolution to defeat this terrorism," he continued.

"It is particularly barbaric this has happened on a day when people are meeting to try to help the problems of poverty in Africa and the long term problems of climate change and the environment.

"Just as it's reasonably clear this is a series of terrorist attacks, it's also reasonably clear, that it is designed and meant to coincide with the G8."

BBC political editor Andrew Marr said Mr Blair was clearly extremely upset when he gave his reaction to the blasts.