A surprise agreed bid by Britain's most profitable (albeit relatively small) supermarket chain William Morrison for rival Safeway, number four in the sector, could be spiked by an even more surprising alliance between Wal-Mart of the USA and J Sainsbury - respectively the world’s largest retailer and Britain's second largest supermarketeer.

The duo are actively considering a joint bid in which the US mammoth – which already owns number three supermarket group Asda – would acquire the lion's share of Safeway’s 479 stores, while Sainsbury takes the rest. A battle royal is likely to ensue which, whatever the outcome, will change the face of the UK supermarket sector for all time.

As yet Tesco, the nation’s largest supermarket group, has made no move.

The Morrison bid, announced Thursday and valued at £2.5 billion ($4.03bn; €3.82bn), has the blessing of Safeway’s board, and shares in the group soared more than 20%, gaining £0.43 to close Thursday at £2.56. But given that Morrison is offering only £2.37 a share, the market clearly believes a counter-bid is on the cards.

The hypothetical carve-up of Safeway has been an ongoing subject of discussion between Wal-Mart and Sainsbury since last autumn, initially on the basis of a 75% - 25% split. The talks ground to a halt, however, when the US giant demanded an even larger share of the spoils.

However, some analysts believe intervention by the duo or Tesco – alongside all of whom Morrison’s is a comparative minnow – would fall foul of the Competition Commission

Data sourced from: Financial Times and Times Online (UK); additional content by WARC staff