Allied Universal said it had received valid acceptances representing roughly 79% of the G4S share capital. The threshold was 50% plus one G4S share.

"My team and I look forward to working with Allied to support a successful integration of the two businesses," said G4S Chief Executive Ashley Almanza.

The merger between G4S and private-equity backed Allied Universal will create the world's largest private security company, generating more than $18 billion in revenue and employing more than 750,000 security guards and other staff.

Sweden's Securitas AB, another big player in the U.S. private security market, will have to compete with an even larger Allied after the acquisition is complete.

Sue Noffke, head of UK equities at G4S shareholder Schroders, said that the company's "management and the board have delivered a very good result for all stakeholders".

Allied, which plans to evaluate options for the G4S prisons businesses, has largely obtained the required antitrust regulatory approvals in the United States and European Union (EU). The deal does not require approval from Britain because EU clearance was obtained before the Brexit deadline.

The interest in buying G4S, which has presence in all major continents, comes after the British company agreed in July to pay 44 million pounds to the UK's Serious Fraud Office to avert criminal charges after a long-running inquiry into electronic tagging contracts.

GardaWorld made its interest public in September last year but was faced with repeated rejections from the London-listed company. Allied swooped in last December and its offer of 245 pence per share won over the G4S board, which recommended that shareholders accept.

Low levels of acceptances from G4S shareholders, however, had forced both potential buyers to extend their deadlines multiple times as shareholders held out for a higher price.

The UK's takeover regulator last month stepped in to help to resolve the impasse by setting up an auction to determine the final prices the suitors were willing to pay for G4S.

But shortly after the auction period started GardaWorld declared its 235 pence per share offer as final, effectively ending the process before it had started.

Shares in G4S, which have risen by more than two thirds since GardaWorld's made its offer public, closed at 244.5 pence on Tuesday.

($1 = 0.7198 pounds)

(Reporting by Yadarisa Shabong in Bengaluru; Editing by Amy Caren Daniel and David Goodman)

By Yadarisa Shabong