The three investors first made an approach to Informa in June, with a potential proposal to buy the company for 506p a share, which valued Informa at £2.15bn.
However, following a period of due diligence, the investment group, which now includes Blackstone in place of Hellman & Friedman, submitted an actual offer of 450p a share yesterday.
But Informa rejected the bid, arguing that the "company continues to have great potential under the leadership of its strong management team".
Derek Mapp, chairman of Informa, said: "The board believes that the revised offer significantly undervalues Informa. The firm has attractive future prospects and is continuing to deliver growth across the business even in the face of a weaker economic environment."
Peter Rigby, Informa's chief executive, said: "Interest in Informa has demonstrated the attractiveness of the company's business model, the quality of its assets and people. The business going forward is well positioned to grow and deliver value to shareholders."
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