G4S has pledged “not to throw in the towel” on its plans to buy ISS for £5.2 billion, despite growing opposition among shareholders who will hold a key meeting this week.

The world’s leading security company needs the backing of three quarters of investors to buy the Danish outsourcer, which specialises in cleaning and catering.

As they prepare for an extraordinary meeting on Wednesday, several institutions have indicated that they will abstain or vote against the acquisition, leading to speculation that G4S might abandon the deal or delay the meeting.