Barclays shares fell for a second day today with investors fearing the implications for the bank of Barack Obama's attack on Wall Street.

The bank — as well as RBS, whose shares tumbled heavily last night — will be exposed to the proposals because of the investment banking business it bought with the purchase of Lehman Brothers' American operations.

The President announced plans last night that would effectively force big banks with retail and commercial arms to break themselves up and limit the scope of their activities to prevent so-called casino banking losses infecting banks that serve the general public.

After tumbling more than 18p a share yesterday, Barclays this morning lost a further 8p to 279p.

London investors and analysts were trying to establish the implications for British firms and the tens of thousands of bankers in the capital working for Wall Street giants Citigroup, JPMorgan, Morgan Stanley and Goldman Sachs.

The initial view was that they would have to split their so-called “proprietary” or prop trading operations — where they trade securities with their own, rather than clients' money — from their other operations. Private equity and hedge fund activity would also have to be curbed.

Mr Obama also wants to curb the size of the positions banks are allowed to take on their prop trading desks, which could severely hit profits but also reduce the risks of massive losses that could once again jeopardise the whole system.

Interdealer broker Icap, which handles a large section of banks' prop trading deals, also saw its shares fall heavily this morning.