Betfair's exchange processes more than seven million transactions a day.
Online betting exchange Betfair has rejected a £912m takeover approach from CVC Capital Partners and other investors.
Betfair said it had received a preliminary bid proposal last week offering 880 pence per share in cash or investments in a new entity.
But the offer "fundamentally undervalues" the company, said Betfair.
Chairman Gerald Corbett said that the company had a "unique business" that "this proposal fails to recognise".
Betfair said it had received the proposal on Friday from CVC together with investors Richard Koch, Antony Ball and partners.
Earlier this month, CVC said it had held preliminary discussions with the investors about a takeover approach.
Mr Koch, a co-founder of LEK Consulting, holds a 6.5% stake in Betfair. Mr Ball is a non-executive director at Luxembourg-listed investment group Brait.
Their preliminary proposal offered 880 pence per share in cash or an "unlisted securities alternative made up of shares and loan notes in a new entity".
But Mr Corbett said: "We have a unique business with a market position, profitability, cash flow and prospects that this proposal fails to recognise."
Betfair's exchange processes more than seven million transactions a day.
Shares in Betfair, which rose 15% last week, closed at 805 pence on Friday.