The UK security firm G4S has rejected a takeover approach worth £3bn from its smaller Canadian rival GardaWorld, which pledged to turn G4S around after a series of scandals over UK government contracts.

GardaWorld said it had offered to pay 190p a share in cash for G4S to create “the world’s leading security service provider” and had made three attempts to engage with the G4S board over the past three months, but that those had “been summarily dismissed or ignored”.

The Canadian firm called on G4S shareholders to ask the board to hold “collaborative discussions” with GardaWorld on a “transaction that would be of clear and immediate benefit to G4S’s shareholders, customers, employees and members of the company’s pension schemes”.

In response G4S said it had also received earlier proposals at 145p a share and 153p a share in June, and had unanimously rejected each of the three approaches. It added: “The board believes that the timing of the proposal is highly opportunistic, coming as it does at a time of severe turbulence in global financial markets.” It urged shareholders to take no action.

G4S shares jumped more than 25% to 182.65p on news of the bid. GardaWorld said the proposed acquisition would be equity-financed by a group led by the investment firm BC Partners, its 51% shareholder.

Founded more than 25 years ago, GardaWorld is the largest privately owned security services and cash-handling firm in the world. It employs more than 102,000 people globally, with 450 offices across 45 countries, and had revenues of CA$3.6bn (£2.1bn) for the year to April.

It has a number of contracts with the UK’s Foreign, Commonwealth and Development Office, and provides security at the British embassies in Afghanistan, Libya, Iraq, Egypt, Malawi, Somalia, Rwanda, Zambia and Tanzania. It also provides security for licensed cannabis producers in Canada.

G4S has 533,000 employees across 85 countries, with its biggest business in North America. It runs cash handling services and security operations, and is also managing 21 Covid-19 test centres in the UK. It runs four prisons in the UK – in Liverpool, near Rugby, Wolverhampton and Wales – but the majority of its contracts are corporate. It provides security at the Hinkley Point C nuclear power station and at Thames Tideway, London’s new super sewer that is still being built.

However, it was stripped of its contract to run HMP Birmingham after rioting forced the government to call in Tornado squads, officers specialising in quelling disturbances. It also lost the contract to run Medway secure training centre, after allegations of mistreatment of children, and the contract to run the Rainsbrook young offender facility. It pulled out of the immigration and asylum sector a year ago, after undercover footage from BBC’s Panorama showed G4S officials mocking, abusing and assaulting detainees at the Brook House facility.

In July, G4S was fined £44m by the Serious Fraud Office as part of an agreement that will result in it avoiding prosecution for overcharging the Ministry of Justice for the electronic tagging of offenders, some of whom were back in prison or found to be dead.

GardaWorld had already approached G4S in the spring of last year, but walked away in May 2019 without making an offer, after speculation about a potential £3bn bid.

Stephan Crétier, founder, chairman and chief executive of GardaWorld, said: “G4S needs an owner, not a manager. GardaWorld has 25 years of experience in the sector and we know how to improve and repurpose this business.

“As owner-operators, we believe that the combined business’s operations will offer a better future for all those who depend on G4S. We will turn G4S around, ensuring it delivers for its customers, its people and the public.”