Construction group Carillion on Thursday piled pressure on rival Balfour Beatty to resume merger talks by revealing that it had held meetings with several of Balfour's major shareholders.
Carillion said it had told the investors that the merger could save at least £175m a year by the end of 2016, enhancing earnings "significantly" from then.
The group had been in talks with Balfour about a merger but Balfour pulled out when Carillion said it wanted it to abandon its proposed sale of US engineering business Parsons Brinckerhoff as part of the deal.
On Thursday, Balfour responded with a statement saying Carillion's latest moves did not address what it described as the "significant risks" of a deal, which it said included doubts about Carillion's proposed savings.
"The board of Balfour Beatty has serious reservations as to the achievability of the stated synergy number," it said, adding that it would make another more detailed statement in due course.
Balfour had said the sale of Parsons was part of its longer-term strategy, but Carillion said it believed it needed to keep Parsons to make the takeover viable.
Carillion said on Thursday that the cost savings of a deal would represent a capitalised value of more than £1.5bn before any re-rating.
It said it had identified savings by cutting back office, head office and other administrative costs as well as applying its own business strategy to Balfour's UK business.
It also proposed to save cash by improving purchasing, standardising IT systems, streamlining the two groups' property in overlapping areas and cutting costs in areas such as use of agency staff, fleet, insurance and general overheads.
Carillion pledged to achieve 40% of the savings by the end of 2015.
It also offered to give Balfour's shareholders an extra cash dividend or equivalent of 8.5p per Balfour share or £59m in total at the time Balfour Beatty's final 2014 dividend would have otherwise been paid in 2015, in addition to the final 2014 dividend they would get as shareholders in the enlarged group.
"Based on initial discussions with banks and assuming the retention of Parsons Brinckerhoff, the board of Carillion is highly confident that £3bn of available funding would be accessible to the combined group," it said.
"Carillion continues to believe in the powerful strategic logic and financial benefits of a merger with Balfour Beatty and is therefore continuing to consider its position.
"Carillion will make a further announcement in due course."
Carillion also said on Thursday that its first-half revenue had fallen 5% to just under £1.9bn, but underlying pre-tax profit had risen 3% to £75.9m.
Basic earnings per share rose to 13.2p from 13p and the interim dividend increased to 5.6p from 5.5p.