The robber made off with foreign currency from Co-op Travel on Lord Street in December.

He threatened the two women members of staff with a hammer and locked them in a back room.

The two women, who were trapped in their own shop, called the police by mobile phone but when officers arrived he had made a getaway.

There has since been a string of other travel agent robberies in Lancashire, Derbyshire and Staffordshire and police believe they could all be linked.

The latest incident was at Thomson's in Stone, Staffordshire, last Monday.

Two women were put in a back room while a robber helped himself to a large quantity of sterling and foreign currency.

Other robberies have been at Colne in Lancashire, Buxton and Littleover in Derbyshire, and Leek and Newcastle-under-Lyme in Staffordshire.

Det Sgt Neil Waterhouse, of Staffordshire Police, said the man, armed with a screwdriver, threatened staff although no-one had been injured.

It is believed he lives outside the areas targeted so far.

The robber is described as white, aged between 40 and 50, around 6ft and with a thin to medium build.

He is also said to have untidy, thinning grey hair and is clean-shaven with gaunt features, a large nose, and a pock-marked face and it is believed he used a silver Vauxhall Zafira during the raids.

Det Sgt Waterhouse said: "We need to identify the offender before he commits another robbery.

"We are working closely with our colleagues in Lancashire and Derbyshire to track him down.

"We hope people will recognise either him or his vehicle."

View police CCTV footage and stills at www.staffordshire.police.uk.
Anyone with information should call 0300 123 4455.