The charges relate to sales to businesses and schools

The Office of Fair Trading alleges collusion on pricing, price rises and arrangements not to compete.

The firms involved are Robert Wiseman, Ballantyne, Graham's, Quothquan, Renfrew and Scottish Milk Dairies.

They have until November to answer the charges. Wiseman and Graham's Dairies have contested the allegations.

If prosecuted, directors could face possible large fines or imprisonment.

The main allegation is that between the years 2000 and 2003 the companies shared price information, co-ordinated a series of price increases and colluded not to compete for each others' customers.

'Third parties'

The alleged infringement concerns sales to what are known as "middle customers" - schools, shops, cafes and hotels - but not supermarkets or doorstep customers.

The OFT has provisionally found that all six firms were involved in price-fixing and all but Ballantyne's were involved in market-sharing.

An OFT spokesman said: "The OFT will not be in a position to decide if the law has been breached until it has received and reviewed the parties' responses to the statement of objections and any comments from interested third parties."

Wiseman will co-operate fully with the OFT and looks forward to a speedy and satisfactory resolution to the inquiry Robert Wiseman Dairies spokesman

Robert Wiseman Dairies said it would "strongly contest" the allegations.

The company generates about �70m a year in sales to the "middle ground" market in Scotland, representing less than 12% of group turnover.

A spokesman said: "We note from the OFT statement that no assumption should be made at this stage that there has been an infringement of competition law.

"This inquiry has been conducted by the OFT over a six-year period. During this time the matter under review has not been put to Wiseman for its consideration.

We strenuously deny that we have breached competition law Graham's Dairies spokesman

"Wiseman will co-operate fully with the OFT and looks forward to a speedy and satisfactory resolution to the inquiry."

A spokesman for Graham's Dairies said: "We have not yet had the opportunity to review the detailed allegations contained in the Statement of Objections which is now being examined by our solicitors.

"However, we strenuously deny that we have breached competition law and we fully intend to make representations to the OFT to refute the allegations."

He added that a similar inquiry was launched in Scotland five years ago but the OFT subsequently decided that there was no case to answer.

Chapter One of the Competition Act prohibits anti-competitive agreements and practices, which prevent or distort competition.