The French group will post its offer document to shareholders on Wednesday, but is understood to have decided not to increase the value of its bid from 720p a share.BPB has twice rebuffed Saint-Gobain. In July it dismissed a 675p-a-share, (£3.4bn) offer, then earlier this month it rejected a revised 720p-a-share bid as undervaluing the company.
The publication of the offer document will set the stage for what promises to be a fiercely fought battle between France's oldest major company and one of the last British leaders in the building materials industry.
BPB is the world's biggest supplier of plasterboard and gypsum plasters, operating in 50 countries and employing 12,500 people. Earlier this month the group promised to return more than £350m to investors and raise its annual dividend by 44 per cent in a bid to fend off Saint-Gobain.
BPB shares closed at 732.5p on Friday, indicating that the market believes Saint-Gobain will have to improve its offer.