Mouchel, which was the target of an unsuccessful takeover attempt by former rival VT Group last year, said Costain’s 105.8p per share proposal significantly undervalued the business.
, which made its approach on 2 December, said Mouchel rejected the all-share offer on 6 December and there were currently no discussions taking place between the pair.
Costain chairman David Allvey said he believed there was a “compelling strategic rationale” behind combining the two businesses.
He said the deal would create value for both companies’ shareholders by bringing together two of the UK’s premium brands in consulting, construction and care, which would have a combined order book of more than £4bn.
He said the deal would create significant savings, would improve earnings and dividend income and would provide the potential for a re-rating of the combined business.
Mouchel’s shares rose 23.5p to 96.5p while Costain’s stock dropped by a penny to 205p.
Mouchel, whose operations range from highway maintenance to consulting for local authorities, has been hit hard by government cutbacks. Earlier this month it embarked upon a strategic review of its business.
Rival support services firm VT Group, which is now part of defence group Babcock, tried to buy Mouchel last year before dropping its interest to try to fight Babcock’s takeover bid.
Costain is boosting consultancy and maintenance operations alongside building and engineering.
At the end of June, 14 per cent of its £2.5bn order book was in operations and maintenance.
Under the terms of the deal, Mouchel shareholders would own about 48 per cent of the combined group.