Belgian brewer Interbrew began a judicial review into the blocking of its takeover of Bass Brewers, branding the decision as "grossly disproportionate" in London's high court.
Interbrew, best known for its Stella Artois beer, bought Bass Brewers last August for £2.3 billion sterling, but the British government ordered it to sell the business in January after a recommendation by the British competition commission.
Interbrew lawyer Mr Jonathan Sumption attacked the commission's report at the start of the high court hearing, which is expected to last three days, with a judgment expected later this month.
Mr Sumption argued that the de-merge order, which the commission claimed was made "in the public interest", was "grossly disproportionate to the supposed disease".
He told high court judge Mr Alan Moses that the commission's report had caused surprise among financial commentators on the basis of the "drastic nature of the proposed remedy".
Interbrew's unconditional Bass Brewers purchase came only months after it bought Whitbread's beer interests last May for £400 million, and it would have given Interbrew a 32 per cent share of the British beer market.