Campaigners have stepped up their fight to help smaller brewers as the number of pubs offering guest beers is set to plummet.
The £400million sale of Whitbread's breweries to Interbrew will mark the end of its pubs' requirement to give access to other brewers' ales under the Beer Orders.
The Campaign for Real Ale has sent a new submission to the Office of Fair Trading (OFT), highlighting the increased risk to smaller brewers who will be denied access to even more pubs.
Without any brewing interests, Whitbread will be able to tie new lessees in its 1,730-strong Pub Partnerships to a specific beer portfolio.
CAMRA predicted this would mean the guest ale right, once available to an estimated 11,000 pubs, would soon apply to no more than a few hundred.
CAMRA research manager Iain Loe said: "The Whitbread sale means that the number of pubs eligible for guest beer will go right down to just hundreds.
"One solution is a change in legislation to make pub chains eligible, although we have been trying to get them to let other beers into their pubs voluntarily."
Consumer groups have been lobbying for revised Beer Orders to force multiple operators to allow a guest ale outside of their supply agreements.
Guest beers are still available to licensees at pubs within Scottish & Newcastle Pub Enterprises and ex-brewery pubs bought by operators such as Punch and Avebury Taverns.
CAMRA is concerned Interbrew may axe Whitbread's cask ales, such as Flowers IPA, Flowers Original, Trophy Bitter and Whitbread Best.
It is also worried about Whitbread's brewpubs, the Lass O'Gowrie in Manchester, the Frog & Parrot in Sheffield and Fellows, Morton & Clayton in Nottingham, which are part of Whitbread's Pub and Bar Company.
Punch Group had to axe the microbreweries within the Firkin pubs that it bought from Allied Domecq last year because they led to the pub operator being classed by the OFT as a brewer. But Whitbread has told its pubs they can continue brewing as normal.
Whitbread continues to own the UK rights to Heineken, which it is reportedly obliged to brew until 2002. It is likely to lease back brewing space at the Magor brewery in South Wales from Interbrew to produce the lager.
But a Whitbread spokeswoman said any arrangements would be "transitional" and the group could not be classed as a brewer.
As The Publican Newspaper went to press, Bass was close to selling its brewing arm for over £2billion. Likely buyers were Heineken, Carlsberg, South African Breweries or, again, Interbrew.