G4S said it had rejected a 1.55 billion pounds ($2.5 billion) offer for its cash solutions business from British private equity group Charterhouse Capital Partners, saying the bid undervalued the unit.
The company, the world's largest security services firm, said on Monday the nature and timing of the non-binding offer, which was made by the group on October 22, was "highly opportunistic".
"The offer has been firmly rejected considering the strategic importance of the cash solutions businesses to G4S and because the Board believes the conditional offer fundamentally undervalues the business and its prospects," it said.
(Read more: G4S to sell businesses, issue shares to reduce debt)
"G4S is committed to invest in its core businesses, including cash solutions, which have strong opportunities for sustainable profitable growth. The cash solutions business is integral to G4S's operations and strategic plans."
The cash solutions unit, whose services includes transporting cash securely, accounted for about 18 percent of group turnover last year.
Charterhouse's offer comes at a time when G4S is trying to recover from a series of damaging incidents with the British government, one of its key clients, the latest being the discovery that it and rival Serco had charged for tagging criminals that were either dead or in prison.