It comes after revelations that a farmhouse with an acre of land was sold between 2001 and 2003 to local councillor Joan Vincent for �7,500.

The land, at Great Treverbyn, was then sold for �170,000 last year.

There is no suggestion the councillor acted inappropriately but the company said it was investigating why the land was sold off so cheaply.

Open sale

County and Restormel borough Mrs Vincent said she became aware several years ago that the company ECLP [now Imerys] was in the process of disposing of surplus land.

After she made inquiries, the company had the land valued and its disposal sanctioned.

She said she acquired one site for �5,000 and the other site for �2,500, which was the valuation in 2000.

Of its sale, she said: "In the last 18 months, due to my husband's ill health, I decided to dispose of the sites on the open market and instructed agents to act on my behalf. The property was subsequently sold."

While there is no suggestion that Ms Vincent acted inappropriately, the Liberal Democrat MP for Truro and St Austell, Matthew Taylor, said his party would monitoring the outcome of the investigation.

Serious issues

He said: "Whether it was sold to a local councillor or anyone else, that's clearly a question that Imerys wants answered.

"And given that the question has been raised it has also been drawn to the attention to the monitoring offices responsible for monitoring the conduct of councillors and others.

"The Liberals Democrats will be looking at it."

Imerys Minerals Limited and the Imerys Group said it took the issues raised by the sales seriously.

In a statement, the company said: "As a result, and in accordance with its group internal policies and procedures, the group is putting in place an internal investigation into these sales."