The company, which has a UK turnover of £919m, disclosed at an extraordinary meeting that it had refused to reopen discussions with Heijmans because talks with Ballast Nedam were too advanced.
This led some shareholders in favour of the deal with Heijmans to threaten legal action.
A shareholders' lobby group, backed by investors holding 35% of HBG shares, said it would request an inquiry by a Dutch corporate court and ask for a court order to stop the deal with Ballast Nedam. The merger must have shareholder approval.
If the Ballast Nedam negotiations do succeed, HBG would own 66% of the new business, with Ballast Nedam owning the rest.
British investor Odey Asset Management has threatened legal action over HBG's refusal to consider a bid for its dredging business from Boskalis.
Heijmans wants to buy HBG's contracting arm, with Boskalis acquiring the dredging business. City analysts still believe Heijmans bid may be successful, even though HBG is three times larger.