Tesco, Sainsbury's and Asda have been blocked from buying rival grocer Safeway. Morrisons has been given the green light to proceed with a bid. BBC News Online examines what the decision means for each of the UK's major supermarkets.
Morrisons stands to gain an instant national supermarket network but the battle isn't over yet.
The competition commission may be convinced, but Safeway shareholders still need persuading.
Some analysts suggested that Morrisons' original £2.9bn all-share offer was not overly generous, and shareholders may feel they are being left short-changed.
That offer - once accepted by the Safeway board and recommended to its shareholders - has now expired and the Morrisons board will have to table a new one.
Morrisons share price has since dropped. Some think a new offer will also be lower than the original, given Morrisons' improved trading position over the summer and Safeway's continued decline.
And while Tesco, Asda and Sainsbury's have been ruled out of the bidding war, Morrisons may still face competition from clothing entrepreneur Philip Green.
Safeway chief executive Carlos Criado-Perez may also offer an alternative to the Morrisons bid - should it be thought too miserly - by proposing a management buyout.
The competition commission has told Morrisons it must dispose of 53 stores, but it will still be able to expand its empire enormously, potentially adding more than 400 Safeway stores to its existing 119 outlets.
A Morrison-Safeway merger would give the combined group a market share of more than 15% - enough to challenge third-placed player Sainsbury's.
Sainsbury's, which has been steadily losing popularity with the nation's shoppers, has perhaps the most to lose from the ruling.
Many analysts believed the takeover of Safeway was the last chance for Sainsbury's to catch up with its rivals in the supermarket league.
Sainsbury's had a firm grip as the UK's favourite grocer until 1995 when it was overtaken by Tesco.
Then the fast growth of Asda - owned by US giant Wal-Mart - further eroded its position and left it trailing in third place.
The latest figures suggest that Sainsbury's had a poor summer - and a Morrisons-Safeway merger is likely to make life even tougher.
Nevertheless, fans of Sainsbury's can take heart that arch-rivals Tesco and Asda have also been left out in the cold.
And some investors will be relieved that the supermarket will not be further distracted from its main task of increasing sales.
The cash and share offer Sainsbury's made for Safeway had been worth about £3.2bn.
At the time, some analysts questioned whether Sainsbury's could handle an expensive and complex acquisition at a time when it needed to focus on rebuilding its existing business.
Tesco is doing well and is unlikely to be unduly worried by the ruling of the competition commission. It remains firmly at the top of the UK supermarket league.
Many analysts believed that Tesco - the last bidder to enter the arena - was not entirely serious about taking over Safeway.
Tesco already has a market share of 27% - a level that meant it was unlikely to have been allowed to buy Safeway. At best, it would have had to agree to sell a large number of stores before taking on its rival.
Observers suggested that Tesco threw its hat into the ring in order to secure a share of any break-up of Safeway.
Tesco has recently announced a big jump in profits.
Analysts believe it will now concentrate on maintaining its recent growth, but it must also ensure that its price reductions do not undermine profits.
Wal-Mart has made no secret of its ambition of dominate the UK market since it bought Asda in 1999.
And, despite the fact that it already controls 17% of the market, it hoped to be able to expand further by buying Safeway.
"I think there was an outside hope that Asda might be
allowed to bid with the proviso that they sold off a large
number of stores. That outside hope... has been completely ruled out," said Richard Prulovich of Gartmore Investment Management.
If it had received the all-clear from the competition authorities, Asda would have had a good chance of winning out - its deep-pocketed parent had the resources to mount an all-cash bid.
Safeway shareholders might have found this more attractive than the prospect of owning Morrison shares.
There had been some speculation that Asda may launch an appeal to the competition authorities or take legal action to challenge the decision, leading to a long, bloody battle.
But, shortly after the decision was announced, an Asda spokesman said it was not currently planning a legal challenge.
Philip Green - the billionaire owner of BHS and Top Shop - has been waiting in the wings for the competition commission's report to be published.
Mr Green's interests are mostly in clothing, not food, so there are no competition considerations and his prospective bid was not scrutinised along with the others.
There has been speculation that Mr Green is no longer interested in bidding for Safeway, but he has denied the rumours.
Mr Green has little experience of food retailing, but has managed to rescue BHS from its frumpy image and turn it back into one of the nation's staple clothing suppliers.
Analysts say Mr Green is in a good position to trump Morrisons' bid - should he wish to.