Right-wing religious leaders and their political allies are hampering the work of experts treating people for diseases such as gonorrhoea and syphilis, the report says.

Professor Anna Glasier of the University of Edinburgh, one of the authors of the report, said: “The bold fact is that the US has such a huge influence around the world. The problem is not so much that conservative forces are growing, but that the countries that have become more conservative are those with huge influence.”

The Vatican also plays a major part in holding back progress, she said.

The report found that each year, 340 million new patients acquire gonorrhoea, syphilis, chlamydia or trichomonas; more than 120 million couples have an unmet need for contraception; 80 million women have unintended pregnancies; and an estimated 19 million women undergo unsafe abortions leading to 70,000 deaths.

Cheap, effective measures could overcome these problems if there was a will to implement them, she said.

On his first day in office, President George Bush reinstated a policy that meant that no US family-planning assistance could be provided to foreign non-governmental organisations working in or providing abortion.

Dr Richard Horton, Editor of The Lancet, said: “For largely political reasons, the US has blocked programmes to save the lives of women from unwanted pregnancy.”

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