The Rail, Maritime and Transport union blames the crisis on a failure by Network Rail to fill vacancies over two years - resulting in a 10 per cent cut in "front-line" technicians and track workers.

With numbers down on the 400 staff the RMT believes are needed, it says the backlog of essential maintenance work in the North-West mounted.

An RMT spokesman, John Tilley, said: "Network Rail are running at what they call deficit manning levels as the norm. The result is that the maintenance people are so stretched, and a backlog has been created."

Network Rail denied that there had been any reduction in the number of maintenance personnel or that a significant backlog had developed.

A spokesman said: "We are making efficiency savings of 31 per cent between 2004 and 2009. There may be a couple of vacancies in the North-West, but this is not a picture I recognise. I am not aware of any particular staffing issues and there is no national policy of natural wastage. We have a to-do list, but nothing I would recognise as a backlog."

The cost-cutting claims came as The Sunday Telegraph discovered that Network Rail, which is funded by the taxpayer and train operating companies, spent more than £40,000 on "sponsoring" events for political parties last year, including a total of £11,000 for a lunch and a reception for Labour MPs.

The company admitted that it spent "£5,000 to £6,000" on seven lunches, dinners and receptions at party conferences.

Yesterday, engineers continued work to remove the Virgin Trains carriages that derailed near Kendal nine days ago, killing 84-year-old Margaret Masson and injuring dozens. An interim report by the Rail Accident Investigation Board blamed a faulty set of points, maintained by Network Rail.