As world leaders finally get around to doing something about international tax evasion, a strange counter argument is forming: tax is a red herring, say the sceptics; better to focus first on sorting out the banking crisis rather than get bogged down in an "irrelevance".

It's no surprise to see people like Martin Broughton say this. As chairman of the CBI (and British Airways), he is paid to deflect criticism from the business community. More worrying is the sight of otherwise sensible commentators making similar points. Martin Wolf in the Financial Times, for example, talks of "consternation" at the European obsession with tax. Carl Mortished in The Times says bank secrecy has nothing to do with the crisis and suggests the clampdown on tax havens is simply due to the wish of our governments to claim ever greater control over national wealth.

While it is true there were many other causes of the financial crisis - too much debt, being the most obvious - it is wrong to pretend that systemic tax avoidance of the sort practiced on a huge scale by our banks is unconnected with their eventual demise. Here are five reasons the two issues should be seen as intimately related.

1) Offshore = out of mind

The offshore vehicles used by banks to hide their activities from the taxman also served to obscure their extent from investors and regulators. Granite, the Jersey-based trust operated by Northern Rock, is exhibit A. Tax havens are also breeding grounds for corruption.

2) Pushing rules to the limit is infectious

As the FSA points out there is no point in having principles-based regulation when you are dealing with people with no principles. Similarly when you encourage bankers to ignore the spirit of tax law and only heed the letter, you encourage them to look for loopholes and shortcuts everywhere.

3) Everyone must pay their fair share

We are all going to have to pay more taxes to clean up this mess, but public support for government bank bail-outs could be fatally undermined if voters believe that only little people pay taxes. The sight of the super-rich swanning about in tax havens should be of more concern to the CBI than anything else right now - without public support, banking is toast.

4) Complexity confuses everyone

One reason why the world of finance became so convoluted was to stay one step ahead of the taxman. The "double dips" and other tax-efficient structures described here ultimately baffled everyone: including their creators. Just ask AIG.

5) Avoidance fuels unsustainable growth expectations

Hiding the true source of your wealth can give your investors a dangerous false sense of security. General Electric, for example, grew lending for years by keeping its tax rate shrinking, until suddenly it lost its credit rating. Far from being "dead money", structured finance vehicles also require large amounts of borrowing to sustain: when the debt market dries up, they are the first to go under.