The Flying Scotsman, the flagship locomotive bought by the London North and Eastern Railway 90 years ago, will need a closer look by private engineers before it can return to the tracks after a lengthy absence, according to an independent report.

The National Railway Museum, which bought the famous train for £2.3 million in 2004, asked First Class Partnerships to review what work might be needed to restore the Scotsman, which returned briefly to the tracks in 2005 before being hampered by engineering problems.

Maintenance costs had reached around £2.7 million, far exceeding the expected expenditure of £1 million. The report suggests that the museum’s internal workshop is “not best placed” to carry out works including the realignment of the middle steam cylinder and the removal of the boiler.

“It was vital that we really got to the bottom of this complex project and received independent verification of the problems associated with the locomotive,” said Paul Kirkman, the Director museum.

“We have now clarified that it is not sensible to complete the work in-house and are in a position to go out to tender for an external contractor.

“We will now progress cautiously towards completing the restoration, subject to reviewing the condition of the main side frames.

“We would like to take this opportunity to thank the public for their ongoing support and patience throughout this challenging project.”

The bid to keep the train in mainline operation – No 4472 is the only surviving example of the A3 class – has been backed by a £1.8 million grant from the National Heritage Memorial Fund and subsequent restoration funding of £275,000 from the Heritage Lottery Fund.

Experts will not be able to fully assess the restoration until contractors have removed the cylinders, revealing a small section of the main side frames which still requires scrutiny. But they said the Scotsman will not operate on the mainline before 2015.

• Read a copy of the report (PDF).