LONDON (MarketWatch) -- The Bank of England on Monday announced details of a plan to swap 50 billion pounds worth of U.K. Treasury bills for high-rated mortgage-backed securities held by commercial banks in an effort to thaw out frozen credit markets. Under the plan, banks can swap assets at any point during the next six months. The swaps are set for a one-year term and can be renewed, at the discretion of the Bank of England, each year for up to three years. The program "is designed to improve the liquidity position of the banking system and raise confidence in financial markets while ensuring that the risk of losses on the loans they have made remains with the banks," said BOE Governor Mervyn King.