Havas Advertising has extended its offer for media buying group Tempus, after receiving a lukewarm response to its £425m bid.

The news increases the chances of rival advertising group WPP winning the battle for control of Tempus.

The French group said it would keep its offer open until September 24, after the previous closing date expired yesterday.

The French group now owns 29% of Tempus, but it has not added any more shareholders to its list of backers since last month.

Analysts said the lukewarm response was understandable, given that WPP has bid 555p for each Tempus share, compared with a Havas offer of 541p per share.

"It's bizarre that Havas have not raised their offer. It seems odd to extend their bid when there is a higher offer out there," said Nicola Stewart, an analyst at Commerzbank.

The development increases the chances of success for WPP, which owns 22% of Tempus.

However, a Havas spokesman said the group has extended its offer in order to let it consider its options.

"We are weighing up our options against the background of deteriorating conditions in the sector. The extension gives us the flexibility we need in an uncertain time," he said.

Many industry observers saw WPP's bid as a cynical attempt to make Havas pay more for its acquisition target, but the French group is unwilling to pay a higher price when the advertising sector is on the brink of an outright recession.

It thought that Havas, previously a very keen suitor for Tempus, is seriously considering walking away from its acquisition target.

Tempus shareholders have until October 1 to accept WPP's offer, while Havas has until late October to raise its bid.