A rail union revealed today it has been campaigning for months to prevent sub-standard contractors from working on track maintenance.

A York-based official of the Rail, Maritime and Transport Union claimed that worrying problems are being ignored by track maintenance firm Jarvis Rail, and that the firm uses "inadequate" sub-contractors.

The alarming allegations come on the day it was revealed that the most likely cause of the London-Leeds train's derailment was a broken track, with Railtrack taking full blame for the fault.

Four people died and scores were injured when a Kings Cross to Leeds high-speed train crashed at Hatfield in Hertfordshire on Tuesday.

Meanwhile, Railtrack's chief executive, Gerald Corbett, will remain in his job after the board rejected his offer to resign following the Hatfield rail crash.

Allen Heath, regional organiser for the RMT union, says contractors all over the country are "playing Russian roulette" with passengers' safety.

Mr Heath says Jarvis Rail - and the industry as a whole - use sub-contractors who are often no more than "jobbing firms" which are less than stringent with safety measures.

Jarvis was fined £500,000 at York Crown Court earlier this year for failing to ensure public safety, after two trains were derailed on lines where it had worked. An investigation revealed both incidents were caused by "track twist".

Since the GNER derailment on Tuesday Mr Heath has told the Evening Press that one of his members has been on sick leave from Jarvis for a year, after he raised the alarm about a faulty track joint on a much-used North Yorkshire line and was overruled by his manager.

Mr Heath says the RMT has been warning Jarvis about using sub-standard workers for a long time, and if the Hatfield disaster turned out to be caused by a rail problem, he will want some serious answers.

He said: "(The derailment) is blowing up an issue that we have been raising - that Jarvis are sub-contracting jobbing firms with no safety requirements. The work being done by some of those sub-contractors is not up to the standards of Jarvis's own staff."

"Following the Hatfield derailment we will be raising matters with Jarvis as a matter of urgency."

A spokesman for Jarvis Rail said: "Jarvis has a rigorous selection, audit and feedback system for sub-contractors to ensure that all work for which it is responsible on the live railway is undertaken to the same exacting standards.

"Since privatisation in 1996 we have been dedicated to achieving continuous improvements in track quality. Our success in achieving this prime objective is clearly illustrated by the significant improvement in all aspects of track quality that influence passenger safety and performance.

"Broken rails are down some 53 per cent over the last two years and there have been no running line derailments in Jarvis maintenance areas during the current year attributable to Jarvis."

Meanwhile, rail services on the east coast main line may not be back to normal until the middle of next week as a result of the Hatfield train crash, Railtrack said today. A spokesman said it was hoped a search of the crash scene would be completed tonight.