The marriage of Ireland's second largest bank and the demutualised building society would create the UK's fifth largest mortgage-lender, with a market capitalisation of more than pounds 11bn. If the talks - which are expected to be formally confirmed tomorrow - are successful, it will increase the pressure on other UK lending institutions to form partnerships.
The merger would also be the first sizeable banking deal in the European Union between a bank from the euro zone (Ireland) and a non-member (UK) of the single currency. Bank of Ireland bought Bristol & West, also a former building society, in 1997 for pounds 600m.
Its desire to expand further into the UK has been thwarted, however, by the reluctance of many building societies to float.
Analysts say a counter bid for Alliance & Leicester is unlikely. Under building society legislation, demutualised building societies are protected from a hostile merger for five years from their flotation, a period which in Alliance & Leicester's case would run until 2002.