The RMT trade union has threatened strike action after two tube train derailments in a 48-hour period.

On Sunday morning the end carriage of a Northern Line train came off the tracks outside Camden Town station, injuring seven people.

London Underground is now conducting an investigation into the incident, which came less than two days after a similar incident on the Piccadilly Line in Hammersmith, and less than a year after the Chancery Lane crash, which resulted in the suspension of the Central Line for 11 weeks.

Bob Crow, general secretary of the RMT, has called for contracts with the consortia charged with maintaining the tracks to be suspended.

"If the mayor of London, transport commissioner or the government do not take action to suspend these contracts, then I shall be recommending to my executives and to the other rail unions on London Underground that we ballot to take strike action to defend the safety of our members and the travelling public," he said.

A spokesman at the union claimed a member of staff had raised concerns about the Camden Town track before the derailment at the weekend.

Speaking to the BBC Radio 4 Today programme on Monday, London mayor Ken Livingstone agreed the incidents raised serious concerns about safety on the Tube.

"These crashes could be just two complete exceptions, or it could be the first indication that the privatisation of Underground management is not working out as we would have hoped it would," he said.

"It might be that this isn't the result of the partial privatisation, it could be that we have just had 19 years of under-investment in the Underground and in many areas, since we have had it transferred to us in July it looks like it has just been held together by a tape and a bit of string.

"But all my instincts are to suspect that this [partial privatisation] may be the problem, that there are too many firms involved, too much delegation, perhaps even a repeat of the mistake that we saw on the overland trains of sub-contractors and sub-contractors sub-contracting the work out.

"Clearly, if it is in any way linked to the introduction of the privatised Underground maintenance, we will be going immediately to the government to ask for the suspension of the contracts and the restoration of a safer regime.

"If you are going to start running three derailments a year, then eventually somebody is going to be killed.

"If there is any evidence that this is caused by the partial privatisation, [transport commissioner] Bob Kiley and myself will be seeing the prime minister immediately to ask for what I presume would need to be emergency legislation to restore safety on the Underground."

However Terry Morgan, chief executive of the Tube Lines consortium which is responsible for the lines concerned, defended his company's actions.

"We have obviously got a lot of thinking and a lot of work to do to understand what caused these incidents," he said.

"These tracks are inspected every 48 hours. Nothing was found.

"We were doing work in that area on the Saturday evening so in addition to inspection we had a team of 20 people down there with a full complement of people who actually checked work after the gangs had left.

"We were aware that this particular area needed a lot of work. It wasn't a safety issue, in fact it was primarily around trying to improve reliability of that area by increasing the maintenance. We have been working in this area since July."