The company's shares have rallied 8% on Tuesday, as some analysts argued that the earlier panic had been overdone.

Adecco's finance director told the New York Times that its decision to put off publication of 2003 results was "not a question of mistakes".

But with the precise background still unclear, many investors remain wary.

The US Securities and Exchange Commission (SEC) has opened an investigation into accounting practices at Adecco's American subsidiary, and rating agencies have now cut their assessment of the company's creditworthiness.

Taking care

Mr Weber was not able to give details of the situation in the company's US unit, which Swiss-based Adecco took over in 2000.

But he insisted that the current situation was not grave.

A recent change of US auditors following the failure of accounting firm Andersen in 2002, combined with new more stringent auditing rules in the US, persuaded the parent company to treat its US results with more than usual caution, Mr Weber said.

As a result, the firm did not feel happy about signing off on its 2003 accounts by the early-February deadline.

In the meantime, Adecco has launched a series of spot-checks across its international operations.

Wait and see

Not everyone has been convinced by Mr Weber's explanation.

"What the market wants is details that are clear and precise," said Thomas Veillet of Swiss investment firm Dynacapital.

Adecco's shares are impossible to value without some indication of the scale of any potential problem, analysts said.

But the broad consensus was that Adecco's woes looked less likely to have arisen from the sort of fraud that lay behind many recent accounting scandals.