Top US banks must raise an extra $75bn

America's largest banks have been told they need billions of extra dollars to shield themselves from a big downturn in the economy.

That's the conclusion of US regulators after they carried out a "stress test" of each of the country's 19 largest banks.

Ten of the banks failed the test and have been told they need to add a total of $75bn to their books to weather the crisis safely.

So, is this the day when the worst is known? One of the key questions about the "stress tests" is whether they were tough enough, and if not, could have they underestimated how much the banks need?

To discuss this, Business Daily's Lesley Curwen talked to Irwin Collier, economics professor at Berlin's Free University.

First broadcast on Business Daily on 8 May 2009