The "Big Six" accountancy firms will be taking a much tougher line on banks' assessments of their own assets in the light of the dubious methods used by some banks to value sub-prime related derivatives.

The six firms - PriceWaterhouseCoopers, Deloitte, KPMG, Ernst & Young, Grant Thornton and BDO - have written a paper due to be published later this month.

It will form the basis for their auditing of bank results for the year to December 31. The paper pulls together all the relevant parts of international financial reporting standards, the rules used by, or being adopted by, more than 100 countries worldwide.

The accountants believe some banks tend to over-value derivatives and other instruments to flatter their own balance sheets, even though the true values of these things have plummeted due to the sub-prime crisis.

They intend to take a "tough line" towards such behaviour, for example demanding persuasive justification where a bank uses "mark-to-model" pricing in instances where less flattering "mark-to-market" pricing is available.

The accountants' paper is intended to allay fears that continuing market turmoil could lead banks and their auditors to reach different conclusions about how instruments are valued and fair prices.

"Given the importance to the market of consistency in times like these, it's good that the large firms are coming together to make sure we have that," commented Jan Babiak, E&Y's managing partner for regulatory and public policy.

Globally, the accountancy profession wants to be seen to be taking a tough stance towards such things to prove its own worth in the wake of the Andersen/Enron affair.

This is one of the themes of World Accountancy Week that starts today. The event celebrates the 30th anniversary of the founding of the International Federation of Accountants, the global organisation for the accountancy profession.