The 62-year-old man had put the sports bag containing rings, necklaces and bracelets on the floor of the Oxfam store in Perth when it was taken. It is believed he may have been followed from a nearby shop where he had been showing the gems to staff.

It is the third time that jewellery salesmen have been the victims of high-value thefts in Scotland over the past 20 months and police are investigating whether there is any link with previous crimes in other areas.

Officers have issued descriptions of a woman and two men who they want to trace in connection with the latest robbery but said they were keeping an open mind as to whether it was an opportunistic crime or was carried out by an organised gang.

The salesman, who is understood to have been involved in the jewellery trade for 40 years and works for a family-run company in London, visited Timothy Hardie jewellers in St Johns Street, Perth, shortly before 3pm on Tuesday.

He then went to the Oxfam shop about 50 yards away on South Street, and put the blue canvas sports bag on the floor while he browsed through some books. The haul is estimated to be worth about £600,000 wholesale, but jewellery experts said the retail price could be as much as £1.5m.

It is believed that a woman may have followed the salesman from Timothy Hardie to Oxfam, where she was seen acting “suspiciously”. The woman, described as between 20 and 25, about 5ft 2in and of “European” appearance, was seen with two men, one of whom was carrying a blue bag shortly after the theft.

One of the men is about 6ft, with thick hair, and of Asian appearance. The other is around 5ft 9in, with thinning grey hair and was seen running across South Street carrying a blue bag, closely followed by the woman. The three suspects then got into a dark saloon car and sped off into Tay Street.

Detective inspector Mike Pirie, of Tayside Police, appealed for witnesses. He said: “We don’t know for certain whether this has been opportunist or whether it is an organised crime group which has targeted the gentleman. We are not ruling anything out.

“Detectives are in the process of examining both public and private CCTV footage from the area in a bid to identify those responsible.”

Several jewellery salesmen working in England have been the victim of organised crime, but such thefts have remained relatively rare north of the border.

Last month a London diamond merchant was attacked outside the Orchard Park Hotel in Giffnock, near Glasgow, and robbed of £100,000-worth of jewellery.

In December 2007, four men stole 800 items of jewellery, worth a total of £250,000, from a salesman as he unloaded his car outside his home in Bearsden, Glasgow. Strathclyde Police said yesterday that no arrests had been made in connection with the robbery.

Meanwhile, a 50-year-old man was held by Flying Squad officers investigating the theft of gems worth around £40m from Graff Jewellers in Mayfair, London, last Thursday. The man was arrested outside a house in east London, on Monday and has since been released on bail.

Two armed men are being hunted in connection with robbery, the biggest gems heist in British history.