Waste management company Shanks has signalled its willingness to accept a £600 million takeover offer.
Shanks confirmed it has received a 135p-a-share approach from a private equity firm, thought to be US buy-out group Carlyle.
The move values the Milton Keynes-based firm at £536 million, but Shanks said that a cash offer of 150p a share - or £595 million - would "deliver an appropriate value to shareholders".
The private equity house is believed to have made three approaches in the last few months, but Shanks did not previously announce them to the London Stock Exchange, believing they were not serious enough, the Daily Telegraph said.
Shanks described the private equity approach as "highly preliminary and unsolicited". It has been in discussions about the approach with its two largest shareholders, Legal & General and Schroders.
Shares in Shanks closed at 90.1p on Friday but were up 40% on Monday at 126.3p.
A successful takeover would make Shanks the UK's third waste management group to fall into private equity hands in recent years, following the buy-outs of rivals Cory and Biffa.
The company was formed in the late 1880s as a construction company based in the west of Scotland. It has UK offices in Lochgilphead in Argyll, Milton Keynes, Southampton, Pontypool, Rainham in Essex and Glasgow.
It raised £66.7 million in a rights issue in May, which it used to strengthen its balance sheet.
Shanks, which employs 4,500 staff, has three key areas of work - recycling, converting waste into energy and Private Finance Initiative (PFI) contracts.