The Flying Scotsman was built in 1923
The return to the rails of the historic steam locomotive Flying Scotsman has been delayed because of problems with its restoration.
The engine was predicted to be back in use this year but railway bosses said it has been put off until summer 2011.
Engineers at the National Railway Museum (NRM) in York are working to restore the locomotive.
The museum said there had been a "series of issues" with restoration which had pushed the timetable back.
The museum bought the locomotive for £2m in 2004 and started a £250,000 appeal to fund the restoration process.
So far just over £140,000 has been raised and work is ongoing.
Helen Ashby, NRM head of knowledge and communications, said: "Some of the components that we've now had the opportunity to strip and check ready for replacement we've found to be more worn than we anticipated.
"It's been a lot more replacement and refurbishment work then we thought there would be originally."
She said to get the loco tested in spring 2011 and fully in use by the summer of that year was a more "realistic programme" as almost all components had now been investigated.
Chris Beet, engineering and rail operations manager at NRM, said once Flying Scotsman was back in full working order, the sense of achievement would be huge.
An appeal was started in 2004 to fund the restoration
He said: "It should have been scrapped 46 years ago by British Railways.
"It's been running longer now in preservation than it actually was when it was built.
"It's a tremendous challenge but it'll be a real achievement for the workshop staff, the museum and for British industry that we can still do work like this 80-odd years after it was first built."
The class A3 Pacific locomotive was built in 1923 for the London and North Eastern Railway (LNER) at Doncaster.
In its career it travelled 2,000,000 miles (3,200,000 km) before being withdrawn from service in 1963.