Gordon Brown is to blame for the recent chaos on a key London Underground line, the Liberal Democrats have said.

The chancellor is coming under fire for the public-private partnership he imposed on the tube network before handing control to mayor Ken Livingstone.

Since then the Northern Line, the busiest line in the capital, has been hit by problems that culminated in a recent two-day shutdown.

Speaking ahead of a London assembly transport committee meeting on Thursday, Lib Dem spokesman Geoff Pope said Brown should be held to account for his part in current problems.

The comments came ahead of a meeting of the assembly's transport committee which will take evidence on the recent problems.

"While many of the tube top dogs are facing the assembly grilling, the one person who has still managed to avoid key questions is the man who set up the PPP in the first place - Gordon Brown," said Pope.

"The Northern Line fiasco shows that the PPP and PFI deals done in the last 10 years on the tube were ill-judged, ill-conceived and poorly implemented.

"It is astounding that after all the chaos that has hit the Northern Line, no one has had the decency to take responsibility for the farce.

"The embarrassing spectacle of each and every organisation playing a game of pass the blame parcel must end."

Meanwhile the RMT union called for a halt to further "fragmentation" of the Underground in the wake of safety problems on the Northern Line and claims that the maintenance company involved may sub-contract work.

General secretary Bob Crow, who is also giving evidence to the committee, will cite the "debacle" over the failure of an emergency braking system on the Northern Line when he calls for Livingstone to be given the power to halt work being subcontracted.

He said the recent trouble should have been "enough to show that fragmentation of tube maintenance is dangerous, and potentially disastrous".

"It is simply staggering that Metronet, which controls two-thirds of London Underground's infrastructure, is planning even more fragmentation, by farming out train maintenance to Bombardier," Crow added.

"That means the same crazy set-up on the Northern Line would be imposed on the Bakerloo, Central, Victoria, District, Circle and Hammersmith and City Lines, with one company running trains, another responsible for overall maintenance and yet another maintaining the trains.

"The government should give the mayor the power to prevent that happening, but the complex, expensive and underperforming PPP itself is at the heart of the problem, and it is time to return all maintenance and engineering work to London Underground."