The news means that Costain could have a much tougher fight on its hands convincing investors to accept its latest £175m offer for the business.

Mouchel’s board hinted that it had already received other approaches late last week when Costain upped it bid for the third time.

The firm advised its shareholders to take no action as it was “actively reviewing” other options.

Capita, Carillion and Balfour Beatty are interested,” one major investor told a Reuters reporter.

“Capita could be keen on buying the whole group but the others might just want specific businesses.”

All three firms have refused to comment about possible approaches.

There is growing speculation in the city that the Mouchel board favours selling parts of the business to raise £60m-plus to pay down debt.

Major contractors like Balfour Beatty are expected to be more interested in Mouchel’s business-process outsourcing capability, which could provide attractive to cash-strapped local authorities.

The report added that leading investors were growing impatient with Mouchel and were pushing to accept Costain’s offer unless a more attractive deal was revealed.

One said: “Mouchel needs to clarify who else they are talking to and what benefits these approaches could bring to shareholders.”

Costain only needs to convince Mouchel’s six biggest shareholders who together hold just short of 54% of the company.

The contractor argues that bringing the two brands together would build a construction business with major capabilities across consulting and construction resulting in an order book of over £4bn.