LONDON (AP) _ A British law against selling beef on the bone went into effect Tuesday, but some butchers said they would flout it and meat inspectors called it unenforceable.

The government announced the ban Dec. 3 after scientists advised there was a slight chance a consumer could catch a fatal human version of the incurable, brain-wasting mad cow disease through bone marrow.

Cuts on the bone, such as T-bone steaks, rib roasts and oxtail, amount to about 5 percent of all British beef sales. Dealers can still sell all their meat, just no bones _ not even for the family dog.

Reports in 1996 of a link between bovine spongiform encephalopathy, or mad cow disease, and a new strain of Creutzfeldt-Jakob Disease, the human equivalent, triggered a crisis in the British beef industry. At least 20 people have died in Britain, and the European Union banned exports of British beef.

British officials told a parliamentary committee Tuesday that the new law would be expensive to police and said only butchers defying it ``loudly and brazenly'' were likely to face prosecution.

Steve Butterworth of the Institute of Trading Standards said his agency had been ``left to enforce the unenforceable'' with no extra resources to meet the increased workload.

But Agriculture Minister Jack Cunningham said he expected the rules to be taken seriously.

Some were determined to ignore the ban.

Ray Robinson, 71, who runs a butcher shop in Burntwood, in central England, said he was prepared to be prosecuted as an ``issue of principle'' rather than stop selling the cuts.

Individually wrapped cattle bones were left outside a butcher shop in Croydon in suburban London, apparently for passers-by to take for the family pet.

``Some people are talking about a black market in beef bones,'' said Gordon Hepburn, chairman of the Guild of Q Butchers.

Britain also faced European Union pressure Tuesday to scrap a threatened ban on beef imports from EU nations that fail to introduce tough new controls to fight mad cow disease.

The EU was investigating to see whether the ban, due to start Jan. 1, complied with the Union's free-trade rules.

Some 170,000 mad cow cases have been found among British cattle, and several hundred cases have been detected in other EU nations.