London mayor Ken Livingstone has been warned not to act "opportunistically" in blaming the Tube's public-private partnership for the latest derailments.

John Biggs, a Labour member of the London assembly, said all sides should react cautiously to the latest accidents.

Speaking to ePolitix.com, the deputy chairman of the assembly's transport committee, said he would aim to "get to the bottom of who was really responsible for the maintenance which appears to have gone wrong on this occasion".

The committee is to take evidence later this week from London Underground, private contractor Tube Lines and union Aslef.

Biggs said he would "keep a reasonably open mind" on what might have caused the derailments. "It is tempting to leap to conclusions, but we don't really know," he said.

But issues set to be reviewed include whether the private consortium has been doing anything new which has inadvertently created a risk, whether they have been cutting corners, and whether there has been a casualisation of staff.

Biggs added the mayor had been "prematurely opportunistic" in blaming the PPP deals for the recent spate of accidents before they have been fully investigated.

And he warned rail companies that they would also be under pressure to learn from recent experiences. "If there are obvious lessons to come out of it then the rail companies would be reckless not to take them into account," he said.