We are surprised and disappointed with this turn of events as we report our progress and activity on a regular and frequent basis to the OFT

Robert Wiseman statement

The others are Graham's Dairies, based in Bridge of Allan, and Hamilton firms Ballantyne and Scottish Milk.

A statement from Robert Wiseman said it had been visited by OFT investigators last week.

"We understand that we are one of a number of dairy companies in Scotland currently co-operating with the OFT's request for information," said the company.

Wiseman said the investigation followed a nine-month Competition Commission inquiry last year, which resulted in no action being taken against the company.

"We are surprised and disappointed with this turn of events as we report our progress and activity on a regular and frequent basis to the OFT," said the company.

No assumption should be made at this stage that there has been an infringement of competition law

OFT spokesperson

"We expect a satisfactory conclusion to this matter."

The OFT confirmed that its searches had involved four Scottish businesses, but did not identify the companies involved.

It said they were part of an investigation into the supply of fresh processed milk in Scotland.

"No assumption should be made at this stage that there has been an infringement of competition law.

"We will not be in a position to decide that until we have all the facts," said the OFT.

The investigation is expected to take several months.

Punitive action

Last year the Competition Commission said that the size of Wiseman dairies was "against the public interest".

But the company was not broken up as a clear majority of the commission did not support the ruling.

Trade and Industry Minister Stephen Byers could only take action against any company if two-thirds of the commission support punitive action.

The four-strong investigation panel were evenly divided on the case and only came up with the verdict on the chairman's casting vote.

The Competition Commission had launched an investigation after allegations of "monopolistic practices" were made against the group in Scotland.