The leader of the RMT has called for a strike ballot over safety fears

London Transport is trying to find out what caused a Tube derailment in which seven passengers were rushed to hospital.

More than 200 people had to be led to safety when a carriage careered off the tracks and hit a wall at Camden Town station in north London on Sunday - less than 48 hours after a derailment near Hammersmith.

Tube managers are investigating reports that the sixth car was wrenched from the rest of the train because of a fault with the track.

London mayor Ken Livingstone said the accidents had sparked grave concern over Tube safety, while the leader of the biggest rail union called for strike action over private maintenance contracts.

It is absolutely critical for those who use the Tube that we get to the bottom of the accident Mike Brown, London Transport

Bob Crow, head of the Rail Maritime and Transport Union (RMT), threatened to recommend a ballot of members unless contracts given to private companies this year were suspended.

He said: "Track walks to check the tracks used to be done every day - now they are only done once a week."

Five of those injured in the accident were discharged from hospital after treatment for minor injuries.

Two others were kept at the Royal Free Hospital in Hampstead, one with a broken thigh and the other with a head injury.

Camden Town station is not expected to reopen until Monday evening at the earliest.

The crash there followed a derailment near Hammersmith station on Friday evening when it is thought a rail broke beneath a Piccadilly Line train carrying 76 passengers, although no-one was injured.

PPP fears

Sources at the train drivers union Aslef say members have complained about track problems at Camden Town and Hammersmith in west London in recent weeks.

Mike Brown, London Underground's chief operating officer, said: "It is absolutely critical for Londoners and for those who use the Tube that we get to the bottom of the accident."

Mr Livingstone said: "We cannot reach firm conclusions until facts are known but one obvious question will be whether, as we feared, the PPP has increased the risk of incidents of this type.

"Londoners and visitors to the city must be able to be sure they are safe on the Tube.

"I have asked London Underground to conduct a full inquiry into the causes of these incidents and what must be done to ensure that they can't happen again."

In January, a Central Line train derailed and hit a tunnel wall at Chancery Lane station, hospitalising 30 passengers and closing the line for months afterwards.

The private company Tube Lines was given responsibility for repairing and upgrading the Northern, Piccadilly and Jubilee Lines this year under the PPP scheme.

The investigation into the two latest derailments will look at the company's practices.