Seven people were taken to hospital after the Camden crash

The announcement comes as London Underground (LU) passengers suffered more disruption when cracks were found on a section of track.

Services between Turnham Green and Richmond, west London, in both directions on the District Line, were halted temporarily after the damaged track was found at Gunnersbury station on Friday morning.

The fault was discovered as the Rail Maritime and Transport union (RMT) said its 9,000 members would vote on whether they want to strike over safety following the derailments last weekend.

Seven people were injured at Camden, north London, on Sunday when the rear carriage of a Northern Line train came off the track and hit the wall.

This will clearly cause disruption to members of the travelling public, which we deeply regret, especially in the run-up to Christmas RMT's Bob Crow

No-one was hurt in a derailment at Hammersmith on Friday evening which has been blamed on a cracked rail.

RMT general secretary Bob Crow said drivers would be asked to reduce speeds as part of a campaign of industrial action which would cause "complete chaos" to Tube services in the run up to Christmas.

Drivers could be told to slow down to just 20mph.

Mr Crow said: "This will clearly cause disruption to members of the travelling public, which we deeply regret, especially in the run-up to Christmas.

"But the message we have received in recent days from members of the public indicate that the overwhelming majority of people share our concerns."

Ballot papers will be sent out next week and the results will be known on 17 November.

Camden Town station is still shut

The union will have to give seven days' notice of any industrial action, so strikes could start on 24 November.

The RMT, which opposed the privatisation of Tube maintenance, says track inspections have been reduced, threatening the safety of staff and passengers.

"The responses we have had from the employers have fallen woefully short of the decisive action we have called for to reverse the dangerous decline in safety standards," said Mr Crow.

The union has been called to a safety summit with LU next week, but Mr Crow said he was not prepared to tolerate any compromise on safety.

LU said it was "very disappointed" at the strike ballot.

"We will be taking a thorough look at our maintenance regimes and practices including the technology, including ultrasonic, that we use to check track for faults."

Parts of the Northern Line will remain shut over the weekend, but LU has said some services should be restored from Monday, subject to safety checks.