This morning’s Stock Exchange announcement did not name the firm.

Costain has now confirmed it is not the prospective buyer after having four bids for Mouchel rebuffed – the latest of which was made last week.

Other suitors previously believed to be interested in Mouchel include Balfour Beatty, Carillion and Capita.

Mouchel said: “The Board is pleased to confirm that it is now in advanced discussions with one potential offeror and has signed a co-operation agreement with that preferred trade purchaser (the “Preferred Possible Offeror”) in relation to a potential offer being made for the Company.”

Mouchel added: “Although the Board of Mouchel considers that the Company has a strong future as an independent business, it believes that the strategic logic of a combination of Mouchel and the Preferred Possible Offeror would provide the best option to deliver value to shareholders.

“The making of any offer by the Preferred Possible Offeror remains subject to a number of pre-conditions, including the satisfactory completion of mutual confirmatory due diligence and the arrangement of financing which is likely to take a number of weeks.”

A Costain statement said: “The Board of Costain notes the announcement by Mouchel earlier today and confirms that Costain is not the party which is in advanced discussions with Mouchel.

“Following the completion of comprehensive initial due diligence, including management meetings with Mouchel, the Board of Costain confirms that it approached the Board of Mouchel on 17 February 2011 with a revised proposal to make a recommended share and cash offer for the entire issued and to be issued share capital of Mouchel.”