The chief executive of Tempus Group has dismissed speculation that WPP will back out of a £432m bid for the media buyer.

Chris Ingrams said WPP has continued to buy shares in his company, despite a freefall in the advertising market following the attacks in the US.

Under takeover rules, WPP can withdraw its bid if there is a "material adverse change" in market conditions, leading to widespread rumours that the advertising giant would drop its offer.

"WPP were buying shares in Tempus on September 17, six days after the attack," said Mr Ingrams.

"They have been buying shares when someone might have argued that there was a material adverse change.

"Also, our latest results prove that there has not been an adverse change in our trading position," he said.

Sir Martin Sorrell's group is the clear favourite to acquire Tempus, after French rival Havas withdrew its bid citing the aftermath of the terrorist attacks in the US as the main reason behind its decision.

The atrocities in New York and Washington have sent manufacturers, consumers and advertisers into shock, leading to a sudden and damaging halt in spending.

But Mr Ingram said the advertising market would pick up in time for Christmas.

"The vast majority of clients know they have to advertise before Christmas, so most of them will press on and get on with it," he said.

WPP has set a deadline of Monday October 1 for acceptance of its 555p-per share bid.

The amount of Tempus shareholders who agree to the bid must be over 90% for the deal to succeed, which leaves both companies in a race against time.