William Hill has already announced its intention to grow the business

The gaming group said the potential buy would delay plans to return �453m to shareholders, a plan announced with its results last month.

A deal would give William Hill almost 25% of the UK's 8,500 betting shops, and could trigger a regulatory probe.

William Hill operates 1,600 betting shops, and also offers telephone and internet gambling.

In London morning trade shares in William Hill were down 14p, or 2.44%, at 559 pence following the news, while Stanley rose 7% or 35p to 507 pence.

There is no certainty at this stage that the full terms of a transaction will be agreed William Hill Check William Hill's shares Check Stanley Leisure's shares

Any deal would take William Hill beyond the 2,000 outlets operated by Ladbrokes.

But it could face competition from other parties, including the UK's Coral, and pooled-betting business the Tote.

William Hill said it was in exclusive discussions with Stanley while it also carried out due diligence on the business, which produced turnover of �1.48bn in the year to May 2004.

"There is no certainty at this stage that the full terms of a transaction will be agreed. No further announcement is expected until the outcome of the discussions have been determined."

A sale of the betting business, which trades as Stanleybet, would leave Stanley with its casino arm, which owns some of London's leading gaming halls and 37 UK sites.

The deal will also fuel speculation that Liverpool-based Stanley Leisure could be moving towards a merger with casino firm London Clubs International.