Two Mercedes - left on Friday outside a club in Haymarket and a nearby street - contained petrol, gas cylinders and nails but the devices did not detonate.

On Saturday a blazing car was driven into Glasgow Airport' s terminal building. Police said it was too early to say if it was linked.

Police in London are studying CCTV footage in the search for suspects.

' Worrying similiarities'

Prime Minister Gordon Brown is expected to chair the third meeting in two days of the government emergencies committee Cobra later.

This comes after police in Scotland confirmed a burning car had been driven into the main terminal building at Glasgow Airport after 1515BST. Two people have since been arrested.

Mr Brown has spoken to Scotland' s First Minister Alex Salmond and Home Secretary Jacqui Smith.

But Downing Street has refused to comment on whether the incidents in Scotland and London are linked, saying this was a matter for the police.

The BBC' s Andy Tighe said there were " worrying similarities" betweeen the two incidents but no-one in authority had yet made any " firm linkage."