Sharp-eyed steam train anoraks can test their knowledge of locomotive engineering, if they buy a new 68p postage stamp.

It has the image of a tank engine -- owned by the Keighley and Worth Valley Railway -- steaming magnificently forward.

According to steam buffs at Haworth Station, where it was photographed as one of a commemorative series of Royal Mail stamps, it is in reverse gear. But the claim is refuted by Royal Mail bosses.

The engine will appear from Tuesday on the highest value stamp to mark the 200th anniversary of the first steam locomotive.

But hawk-eyed steam lovers who had a sneak view of the stamp, spotted the alleged error and had to smile. "It's in reverse gear yet it looks like its going forward because of the direction of the smoke," said volunteer driver and fitter Ralph Ingham.

"Most people won't spot it but somebody with a technical knowledge -- an anorak -- will notice it."

But there were no complaints and the railway felt proud and delighted that the engine had been chosen, he added.

A Royal Mail spokesman said: "We have spoken to the designer, who was at the photo shoot staged in preparation for the design of the stamp and the locomotive was definitely going forwards. We are pleased, however, that people do examine our stamps so closely."

It was the first time a K&WVR locomotive had been featured on a Royal Mail postage stamp.

The engine, built in Derby in 1952 and bought by the K&WVR in 1969, was recently seen on the Christmas edition of BBC's Songs of Praise, presented by TV celebrity Pete Waterman.

The British Standard Class 4MT engine pulled carriages along the line from the early 1970s before it was sidelined for boiler repairs, only appearing again after 20 years.

It is now one of the major locomotives on the five-mile line between Keighley and Oxenhope.

The six stamps -- 20p, 28p European Class, 42p, 47p and 68p -- commemorate the launch, 200 years ago, of the first ever steam locomotive, the Pen-y-darren, designed by Richard Trevithick, a pioneer of steam locomotion.

The engine, based in Wales, was able to haul 20 tonnes of iron.