The Conservatives and Liberal Democrats have urged the London mayor to act on problems with the tube network.

An emergency debate in the London assembly was on Wednesday set to hear calls for Ken Livingstone to take action on the Northern Line's technical difficulties.

The session was called after recent problems led to severe delays on the line.

Conservative assembly members were seeking to force the mayor to produce an urgent plan of action to resolve the ongoing disruption.

Under a rarely used power, the assembly can oblige the mayor to report to it on a specific issue.

Assembly Tory Brian Coleman said: "We need to do everything we can to get this situation resolved. That is why we are using this rarely used power to force the mayor to report to the assembly on this urgent matter."

The Conservative call is contained in an emergency motion they will propose at the assembly meeting.

The motion also welcomes a decision by London Underground last Friday to issue an emergency direction to the contractors allegedly responsible for the safety systems that failed.

The Lib Dems in the assembly also welcomed London Underground's move but intend to question whether the move was robust enough to prevent further delays to passenger journeys.

They say that the "emergency step in powers" should be applied now.

Lib Dem assembly transport spokesman Geoff Pope said: "Enough is enough on the Northern Line.

"Passengers using this crumbling misery line are at the mercy of a poor PFI deal and a PPP contract that has not delivered. The state of the line has been raised time and again with the companies who maintain it, yet things have only gone from bad to worse.

"London Underground should exercise its step in rights and take control of the repairs whenever passenger safety is put at risk."