LONDON (AP) — Prime Minister Gordon Brown summoned the head of Britain's central bank and chief financial regulator for talks yesterday after banks' share prices plunged amid investors' worries that the government hasn't done enough to strengthen bank balance sheets.

Brown and Treasury chief Alistair Darling are holding a meeting with Bank of England governor Mervyn King and Financial Services Authority chairman Adair Turner to consider possible new action to stabilise the country's financial system.

Brown's spokesman Michael Ellam declined to say what specific measures the meeting will consider, or whether the men will discuss a possible plan for the government to use taxpayer money to buy large equity stakes in British banks.

Shares in the Royal Bank of Scotland Group PLC, whose credit rating was downgraded by Standard & Poor's credit agency on Monday, plummeted by 39 percent to close at 90 pence ($1.58) yesterday in London. HBOS PLC closed down 41.5 percent at 94 pence ($1.65), Lloyds TSB Group PLC dropped 13 percent to 225 pence ($3.96) and Barclays PLC fell nine percent to 285 pence ($5.01).

The decline in bank shares followed a meeting on Monday between the government's leading financial figures and the heads of Britain's biggest banks, during which they discussed a possible rescue plan for the beleaguered banking sector, according to two banking officials with knowledge of the meeting who asked not to be identified because the meeting was confidential. Darling, King and Turner met with heads of RBS, Barclays and Lloyds to discuss the possibility of the government investing as much as £50 billion ($87 billion) in the country's banks in order to shore up their balance sheets and restore confidence in the institutions, the banking officials said. The £50 billion in taxpayer money would buy the government large equity stakes in the banks.

But the two people familiar with the meeting said the government did not give details of exactly how much it would invest or when the proposed plan might come into effect.