G4S has rejected a £3.3bn takeover offer from US rival Allied Universal in the latest twist in a bidding war for the security services firm.
The 210p-a-share offer “significantly undervalues G4S and its prospects”, the FTSE 250 company said on Tuesday.
The “highly conditional indicative offer” from Allied, backed by UK private equity firm Warburg Pincus, was made last Wednesday.
News of the offer was released "without the consent of Allied", G4S said, and Allied failed to respond to requests for comment about it.
The board of G4S, led by City veteran John Connolly, has been fending off a 190p-a-share hostile swoop by Canadian firm GardaWorld since September.
Garda, owned by European buyout firm BC Partners, has clashed with G4S in recent weeks.
Boss Stephan Crétier has said the G4S board had “failed to deliver” over the past seven years and has warned shareholders that a five-year plan to turn the business around “defies rational analysis”.
Mr Connolly hit back, saying that Garda “desperately needs G4S” as part of an M&A splurge.
On Tuesday Mr Crétier questioned Allied's approach.
"Any sophisticated shareholder will immediately see the insuperable anti-trust issues Allied would face in the US," he said. "Unlike Allied, which has made a ‘highly conditional indicative offer subject to substantial due diligence requirements,’ our financing is real, our offer is a matter of public record and our intent is serious."
He stepped up his attacks on G4S, saying the board was "shadowboxing" by revealing the Allied offer and should "without delay engage with us in the best interest of shareholders".
Three of G4S’s biggest investors - Schroders and US investors Harris Associates and Sachem Capital and whom between them control 25pc of G4S - have publicly rejected Garda’s offer, but said that they are open to a higher bid.
All three declined to be drawn on whether Allied's price was sufficient to get them to engage.
The Telegraph reported on Oct 21 that Allied was poised to lodge a rival bid for G4S after registering an expression of interest two weeks earlier.
GardaWorld has given investors until Nov 7 to indicate whether they will accept its offer. This deadline could be extended, however.
G4S said it had hired external consultants so that it could share confidential information with Allied, GardaWorld and any other potential bidders that may emerge in the coming weeks.
Shares closed 3.1pc higher at 211p.