Shares in media buying group Tempus have fallen by nearly 25%, amid concern that WPP chief executive Sir Martin Sorrell may pull his 555p-per-share bid for the company.

The board of Tempus formally backed WPP's bid today after rival suitor Havas backed out, citing as reasons the stock market collapse and the effects of the US terrorist attacks on the advertising market.

But the company's plummeting share price suggests investors are betting that WPP will be able to get out of its obligation to pay 555p for Tempus shares, way above their true value.

Tempus, which had proved largely resistant to the global collapse in share prices, saw the value of its shares drop by 130p from a high of 555p earlier today.

Lorna Tilbian, a media analyst with brokers Numis, believes the current political situation may enable WPP to extricate itself from the takeover bid.

"The market seems to be betting WPP will be able to get out of this, and the market is seldom wrong about these things," she said.

"Martin did not initiate this, he responded to Havas's offer. He got himself into a win-win situation, but now things have changed he can walk away without losing face."

If WPP does pull out, it will have to wait another six months before submitting a new bid under the takeover code.

Ms Tilbian speculated this would provide time for Tempus's share price to fall further, allowing WPP to make a much lower bid.