A CLASSIC steam engine will not return to main line service until a year later than planned following a major overhaul, it was announced today.

A shortage of raw materials has been blamed for the hitch affecting Flying Scotsman, saved by the National Railway Museum at York from being sold abroad three years ago.

The locomotive, which once hauled expresses between London, the North-East and Scotland, is in pieces as experts work on it to extend its 83-year life for the latest generation of rail enthusiasts.

It was originally envisaged that work on the engine, the flagship of the NRM, would be completed by the end of this year with a return to service in January 2008.

Following discussions between the NRM and contractors, however, the museum has announced an estimated completion date of December 2008, with the locomotive ready for running again in January 2009.

The NRM, which is overseeing the overhaul, said it had been discovered that the engine's firebox needed more extensive work than first anticipated but there had been a UK shortage of the special type of copper involved.

The material had now arrived and was ready for pressing by a specialist firm in Devon.

Helen Ashby, head of knowledge and collections for the NRM, said: ''There has been a delay to the overhaul, but it was crucial that we had the correct copper and we have also been very keen to keep the work in the UK.'' Work on the wheels and chassis is under way at the NRM workshops while the boiler has been sent to another specialist firm.

Delays in building an Edinburgh visitor centre for the engine led its previous owners to auction it in 2004. The NRM's successful sealed bid included £365,000 raised through a public appeal, matched by a donation from Sir Richard Branson on behalf of the Virgin Group. The National Heritage Memorial Fund stepped in with a £1.8m grant.

The NRM then ran the engine on special trains between York and Scarborough.