“The combination of G4S and Allied Universal creates the global leader in security with over 750,000 employees, industry leading capabilities and unrivalled market coverage,” said G4S Chief Executive Ashley Almanza in the statement.

“This unique and compelling combination will offer customers exceptional service and provides employees with an exciting future.”

The London-listed company had already turned down a £3.25-billion proposal from Allied in early November, arguing that this conditional approach had undervalued its prospects.

Steve Jones, president and chief executive of Allied Universal, added on Wednesday that the group was “delighted” to win the board’s recommendation.

“Our businesses know each other well, we share a similar culture and values and I am excited about what the combination of Allied Universal and G4S can deliver,” he said.

“G4S has an excellent service offering, an enviable global customer portfolio and it is led by a highly experienced management team.”

G4S, which itself employs 533,000 staff across 85 countries, had faced a takeover tussle between Allied and GardaWorld with both suitors pledging to vastly improve its tarnished image.

While mostly linked to allegations of mistreatment by staff in the UK, the company has been accused also of mismanagement at the US military’s Guantanamo prison camp and in the West Bank.

The company has become an attractive buying opportunity after its share price hit a 16-year low earlier this year.