That RBS is set to launch Britain's biggest rights issue will come as no surprise to City insiders. Analysts had been anticipating the cash call for weeks given the low level of the bank's capital cushion compared to its peers.

But those relying on public statements from RBS itself will have every right to be shocked.

Only in February Sir Fred Goodwin, the bank's beleaguered chief executive, vowed that RBS had no need to raise fresh capital. “Not us!” he declared. “No way! Not now!”

Yet weeks later, Sir Fred is knocking on the door for nearly £13 billion.

It is difficult to swallow the line that March was simply the most devastating month ever.

By analysts' calculations, RBS had about £13.5 billion of remaining exposure to leveraged finance, £3.8 billion to collateralised debt obligation and £1.1 billion to US sub-prime in February.

To be sure, those figures are unpleasant, but they are hardly catastrophic - certainly not compared to the mind-boggling exposures held by some of Wall Street's finest.

Writedowns alone are unlikely to have triggered such a desperate need for capital.

When capital-eroding writedowns come at the same time as weakening markets, RBS effectively is hit by a double whammy, since in troubled markets banks need to keep even higher levels of capital for the sake of prudence.

It is telling that RBS's move comes in the same week that banking chiefs, including Sir Fred, went with their begging bowls to No10 to persuade Gordon Brown and the Bank of England to pump additional funding into the money markets.

Without it, they warned, Britain's mortgage lending would be cut in half, having a dire knock-on effect to an already creaking economy.

The Prime Minister has made clear that he will not bail them out without serious housekeeping on the banks' side, too.

He wants them to come clean on their potential losses and bolster capital levels to provide more security.

In that respect, RBS's move looks less clumsy and other banks, rather than playing down speculation that they, too, have rights issues in the works, would do well to consider getting the news out sooner rather than later.

Shareholders may yet thank them, not to mention the rest of us who may benefit as banks start to free up cash for cheaper mortgages.

The American banks have gone on a capital-raising binge in recent weeks while Britain's banks have stuck to a diet of bread and water.

It's about time they got stuck in.

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