The 'will they, won't they' saga has been rumbling on now since the pair revealed they were in talks last month.
But Balfour rejected a first proposal and there has been a continuing debate over the future of Balfour's engineering and design business Parsons Brinckerhoff.
Today, it clearly repeated its doubts over the union, which Carillion said yesterday could reduce the combined cost base by at least £175mln a year.
Among its arguments today, Balfour said the combined group would be of a significantly larger scale and diversity than the Carillion management team has previously managed, with annual revenues of around £14 billion and around 80,000 employees, excluding joint ventures.
The proposed retention of Parsons Brinckerhoff exacerbates the scale of the challenge at a time when the management team would be undertaking a fundamental downsizing of the UK construction business, it added.
Balfour also claimed that Carillion's proposals would reduce revenues at its UK construction arm by two-thirds.
This division generated £2.8bn of revenue in 2013 and Balfour Beatty claims a "significant reduction in overheads" would, therefore, be required in order to maintain profit margins at the business.
The implementation programme would be complex, requiring simultaneous business restructuring, integration and outsourcing, it also said.
"The proposal remains unchanged to that rejected on 11 August 2014," it told investors.
"The board believes this is the right time to sell Parsons Brinckerhoff, but believes Carillion's approach for the entire group at this stage of the construction cycle is opportunistic."
Balfour shares rose 0.5% to 241.30p.
Carillion shares eased 2.34% to 338.7p.