Britain faces a potentially serious shortage of medicines because the plunging value of the pound has made it more expensive to import drugs and attractive to export them.

Imports of medicines into the UK from Europe have fallen sharply as the drop in the value of sterling, down 25 per cent against the euro since the summer, has wiped out traders' profit margins. Meanwhile, exports have risen as traders capitalise on low UK prices to seek new markets abroad in Scandinavia and central Europe.

Major pharmaceutical companies including Pfizer, the maker of Viagra, and AstraZeneca, who make the cholesterol-lowering statin Crestor, have begun rationing the quantity of drugs released to British pharmacies, GPs and hospitals to prevent their being re-exported abroad.

Some NHS patients have had to wait days to receive their medicines while pharmacists negotiate with manufacturers and wholesalers to release the drugs. David Pruce, director of policy at the Royal Pharmaceutical Society, said: "We haven't yet reached the stage where there are widespread shortages, and there is no need to panic. But it could turn into a crisis and we need to keep a close eye on it ... The Government needs to work with the pharmaceutical industry and wholesalers to ensure supplies are maintained."

An estimated 12 per cent of drugs in the UK, worth more than £1bn a year, are brought in to the country after being purchased in other countries in Europe by "parallel traders", small wholesalers who trawl the continent looking for bargains. When the pound was strong, these traders bought drugs in Greece and Spain for sale in the UK, where prices were higher. Now, traders are buying products in the UK,repackaging them and selling them in Germany and Scandinavia.

In the 12 months to November 2008, licences granted to parallel traders to import drugs fell by 60 per cent, according to the Health Service Journal. The value of exports rose 27 per cent compared with a year earlier. In the last four months of 2008, the Medicines and Healthcare Products Regulatory Authority issued 41 per cent more export certificates than in the same period of 2007.

Martin Sawyer, executive director of the British Association of Pharmaceutical Wholesalers, said: "There is a lot of evidence of British products in the Scandinavian countries and in Germany." There was a danger "the UK could become the new Greece or the new Spain", he said – a reference to the shortages these countries have experienced in the past when targeted by traders seeking low-cost drugs.

The Department of Health said it was in regular contact with the drug companies and wholesalers and was "monitoring the situation closely". Major drug manufacturers have begun monitoring UK pharmacies for unusually large orders that might suggest they are selling drugs on into Europe.