Odey Asset Management, for example, holds a short position in UK mortgage lender Bradford & Bingley, according to one of the first filings under new rules set by the Financial Services Authority, the UK's financial regulator.

London-based Odey said it held a short position of 0.28 per cent in the mortgage lender, which has seen its shares plunge by as much as 75 per cent this year, according to Bloomberg.

The FSA imposed new rules on 20 June that require investors to reveal short positions of more than 0.25 per cent of stock for companies that are selling new shares in rights offerings.

The FSA has blamed short bets earlier this month for "severe volatility in the shares of companies conducting rights issues." That followed a slump in shares of Royal Bank of Scotland Group, the UK bank, and HBOS, the country's largest mortgage lender, as they tried to raise a combined £16 billion ($31 billion) of capital.

The Alternative Investment Management Association, a trade group for the hedge fund sector has complained that the new rules have been introduced without consulting market players.

Meanwhile, the American hedge fund group Harbinger Capital Partners revealed that it has made a significant bet on HBOS's price falling, while its UK counterpart GLG admitted it is targeting mortgage bank Bradford & Bingley, as investors were forced yesterday to disclose their short positions to the market for the first time.

Harbinger Capital Partners, which is run by the former Barclays Capital trading boss Philip Falcone, revealed that it held 3.29 per cent of HBOS's market capital on loan.