Nathan Ford of Médecins Sans Frontières put the cut down to the effect of competition from generic companies. "It is important to point out that this wouldn't have happened in a vacuum," he said.

GlaxoSmithKline, the British pharmaceutical company, has slashed the price of its Aids drugs to sub-Saharan Africa and the rest of the world's poorest countries, it announces today.

The cut almost halves the annual cost of treatment with its drug Combivir to £206 , which is within sight of the low price offered in the past couple of years by the generic "copycat" companies in countries unconstrained by international patent laws.

Ranbaxy in India offers a World Health Organisation-approved version of the same combination of Zidovudine (AZT) and Lamivudine (3TC) for £167 a year. Aurobindo's version, without the WHO stamp of approval, costs £128.

GSK will be hoping that the UN Global Fund for Aids, TB and Malaria, which is raising money for poor countries to buy drugs, will now allow that money to be spent on Combivir.

The fund has said that its stretched resources have to be spent on the cheapest good-quality drugs.

GSK may also have an eye on the new money for Aids announced by George Bush in his state of the union address. When he promised $15bn (£9.4bn) for Aids, of which $10bn was new money, he referred to Aids treatments costing less than $300 (£189) a year.

At the time only the generic companies sold their drugs at such low prices.

Jean-Pierre Garnier, GSK's chief executive, emphasised that this was not its first price cut. "These price cuts demon strate our commitment to making vital medicines more affordable through sustainable preferential pricing," he said.

"In June 2001, when we expanded our access programme, we promised to continue to find ways to reduce costs and pass those savings on to patients. We did that in September 2002, and today we are again delivering on our promise."

Campaigners welcomed the decision but questioned why GSK was cutting its prices so dramatically now.

"What has happened to their existing offers?" Sophia Tickell of Oxfam said. "I think we will find not very much when there are generics being offered at lower prices."

GSK said it had trebled sales from 2 million people treated to 6 million in the 63 poorest countries between 2001 and 2002. Sales had not increased much since the last price cut, but manufacturing processes had improved.

Nathan Ford of Médecins sans Frontières put the cut down to the effect of competition from generic companies. "It is important to point out that this wouldn't have happened in a vacuum," he said.

Meanwhile the European commission has been accused of giving in to pressure from pharmaceutical companies in preventing a US expert from addressing developing countries on cheap drugs.

James Love, of the Consumer Project on Technology, which was set up by the radical American campaigner Ralph Nader, had been invited to a Beijing conference on patent rights and public health.

"I was told by the Chinese government that the European commission objected to my participation in the programme," he says in a letter to Pascal Lamy, the EU trade policy commissioner.

He was replaced by an adviser to US drug companies. Mr Love also accused the US government of putting pressure on UN agencies to prevent him offering technical advice to poorer countries.

"I assume that the commission and my own government are doing this at the request of the European and US pharmaceutical industry," he says in his letter, passed to the Guardian. "I can only imagine why the European and US governments believe developing countries should not be permitted to hear from experts of their own choosing."

Pharmaceutical companies agreed to allow the use of cheap copies of anti-Aids drugs in the poorest countries under the World Trade Organ isation's Doha declaration, which also applies to diabetes, asthma, cancer, heart disease and a number of other afflictions.

At the heart of the issue is a technical instrument called "compulsory licensing", which enables national public health objectives to override patent interests. The legal implications have still to be explored.

Mr Love meets the Brussels commissioners to discuss Doha today. An international meeting in Geneva tomorrow will tackle the problems of financing research into the illnesses of the developing world.