The deal, sealed during Prime Minister David Cameron's three-day visit to China at the head of a trade delegation, follows a similar agreement last year on the export of live pigs.

The industry has been lobbying for the inclusion of semen on the grounds that the Chinese pork producers they deal with prefer to source it from the same country where they obtain sows for breeding.

Four artificial insemination centres operated by some of the world's biggest pig breeding companies in England and Northern Ireland are expected to start exporting fresh and frozen semen early in the new year.

Half the world's pigs are in China, and improving pig genetics is key to China as pig meat is one of their main sources of protein. China wants to increase the efficiency of its production, whilst minimising the environmental impact.

It is hoped that the UK industry can play an important role in helping China achieve greater efficiency through the provision of high quality genetic stock.

Britain is lobbying Beijing to also be allowed to export pig trotters, which are a waste product in the UK pork industry but are widely eaten by Chinese diners.

Exports of trotters could be worth as much as £7.5 million a year to the industry in Britain.

A Downing Street spokesman said: "We're doing all we can to ensure that businesses up and down the country reap the rewards from our relationship with China. And that includes our pig farmers.

"This new deal to export pig semen will be worth £45 million to UK firms and means Britain's best pigs will help sustain the largest pig population in the world.

"And we're not stopping there - we're talking to the Chinese about serving up pigs trotters on Beijing's finest dining tables. That would be a real win-win - a multimillion-pound boost for Britain and a gastronomic treat for Chinese diners."