The Authority for Financial Markets said it has levied the fine on Dragados and HBG for not informing shareholders of takeover talks at an earlier stage. Both companies have denied the allegation and say they will appeal. The fines are largest that have ever been publicly imposed by the AFM.
Dragados announced details of its bid for HBG on 5 February but the AFM said the announcement was too late for investors. It believes that by then the companies had already reached a point in the negotiations at which they should have informed shareholders of the deal.
The AFM said its findings would not affect the takeover, which was to be finalised this week.
A joint Dragados/HBG statement said: "Dragados and HBG are of the opinion that they did not breach the provision, so that there is no grounds for imposing the intended fines.
"If the AFM nevertheless goes ahead with the imposition of penalties, Dragados and HBG will appeal."
HBG says its UK business, which has a turnover of £630m, is unlikely to be affected by the takeover.