The revelation casts doubt on GNER's ability to operate if Sea Containers becomes insolvent.

Sea Containers - which has its headquarters in London but is listed in New York - warned this week that it expected its auditors to question whether it was a going concern.

The company had recently insisted that its problems had little impact on GNER's contract to run trains on the London to Leeds and Edinburgh route.

But, in its annual report to the SEC last year, Sea Containers revealed that it provided a $55m (£30m) stand-by credit facility to GNER before May 1 last year, when it started its latest, 10-year operating franchise.

The facility's existence was a condition of the award of the new franchise, according to SEC filings. If it were withdrawn because of Sea Containers' problems, GNER could be in breach of its franchise conditions.

Sea Containers also guarantees a £10m working capital facility for GNER and a performance bond with the Department for Transport - now in charge of rail franchising - which is also vital for GNER to continue services.

Sea Containers played down the importance of the credit facilities, saying GNER was able to finance its own operations. The DoT said GNER was still satisfying all its franchise obligations.

GNER is also facing other problems, since in March the Office of Rail Regulation unexpectedly refused its request to run extra services and instead let a new operator run services which will take some GNER revenue.

The decision seriously affects GNER's ability to make £1.3bn of payments to the government under the franchise agreement.