Yet another huge loser in the American subprime mortage market is set to announce big writedowns next week.

Royal Bank of Scotland (RBS), Britain’s second largest bank, is understood to be seeking to raise capital from its shareholders in a rights issue thought to amount to £10 billion ($20bn), which is probably the biggest rights issue ever demanded in the UK.

The bank, which bought troubled NatWest and ABM Amro, has been running on low capital ratios for quite a while. It also has major exposure to subprime debt instruments. It has been linked with Spain’s Banco Santander for many years.

When such a major player is caught short like this, it brings home the extent and depth of the crisis in transatlantic financial markets, with all the knockon effects to the rest of the world.

Vince Cable, a spokesmen on Treasury matters who carries more weight than the Treasury these days, believes all the banks should follow the example of RBS, since they will need a great deal of liquidity from the Bank of England and that should be underwritten by shareholders, not taxpayers.

We await next week’s announcement, which will surely be leaked over the weekend.