The City watchdog has renewed its warning to consumers to be wary of getting caught up in share scams amid fresh evidence of a sharp rise in “boiler room” activity.

Boiler rooms are so called because they involve high pressured selling techniques to encourage UK consumers to invest large sums of money on worthless shares.

Boiler rooms are notoriously hard to monitor and close down as they are often based offshore in places such as continental Europe, Asia and the US and are not authorised by the UK authorities.

The Financial Services Authority has for a number of years logged the calls made to its help-line from consumers worried about boiler room approaches.

According to its latest data, the watchdog reported a sharp rise in the number of calls to its helpline in October, with 63 calls in the first eight days of the month.

No other data is yet available for the remainder of October, but the 63 calls alone rank the month among the busiest of the year.

“The numbers were surprising and may reflect that boiler room activity had been particularly active at that time,” says an FSA official.

“However, there is very little we can do to stop the boiler rooms from operating. All we say to people again and again is to report the boiler rooms so we can put them on our list.”

The fresh warning came as it emerged that boiler rooms were using new tactics to target investors.

Diageo has alerted the FSA after some of its investors received cold calls from “fast talking” individuals claiming to represent the global drinks group.

The agents, said to be based in the US, tried to persuade the shareholders to sell their shares and invest the proceeds in other investment schemes.

“We believe these individuals got the information about our investors from an out-of-date register of shareholders,” says Diageo.

“We have written to 110,000 of our shareholders alerting them that these individuals are not connected with Diageo in any way.”

The Diageo case comes as victims of a boiler room scam dating back to 2000 were given some hope of recovering lost funds.

Last week the Court of Appeal ordered a UK-based solicitor and a London-based law firm, which acted as front for an overseas-based boiler room, to pay £360,000 to 63 investors stung in an investment scam five years ago.

The Spanish-based boiler room operated by making unsolicited telephone calls to UK residents during which agents recommended investors purchase certain shares, at inflated prices, on the promise that they were going to rise substantially.

More than three-quarters of a million pounds was invested into the account of the UK solicitors, which acted as a “respectable” front for the Spanish boiler room.

At the end of the month, the FSA had 207 names on its list of unauthorised firms operating in the UK.

It advises consumers to keep checking the list regularly as firms often change their names.