By Ben Hirschler

LONDON (Reuters) - GlaxoSmithKline plans 275 million pounds ($361 million) of new investments at three drug manufacturing sites in Britain, signaling its confidence in the country despite last month's vote to leave the European Union.

The decision came as Britain's biggest drugmaker reported better-than-expected quarterly earnings on Wednesday, buoyed by new drug sales and currency gains from a falling pound, which it said could pump up full-year results by 19 percent.

GSK, which had argued against Brexit before the referendum, said the UK remained an attractive place for making medicines, thanks to a skilled workforce and relatively low tax rates.

The country's so-called patent box boosts profits from patented innovations by halving the rate of corporation tax. This tax relief, which favors pharmaceutical companies, has come under fire in recent days from the opposition Labour Party.

GSK is investing in sites at Barnard Castle, in the north of England, Montrose, in Scotland, and Ware, north of London, to increase production of next-generation respiratory drugs and biotech medicines.

The vast majority of these products will be exported.

"It is testament to our skilled UK workforce and the country’s leading position in life sciences that we are making these investments in advanced manufacturing here," said Chief Executive Andrew Witty.

Business minister Greg Clark said GSK's move was a clear vote of confidence in Britain and demonstrated that "there really is no place better in Europe to grow a business".

Witty, who is retiring next year, said Brexit could disrupt Europe's unified system for drug regulation and potentially hamper access to top scientific talent, but this was not enough to offset Britain's fundamental competitiveness.

HELPED BY STERLING FALL

Britain accounts for nearly half of GSK's worldwide research and development and around a third of its manufacturing.

Uncertainty surrounding the vote to leave the EU in last month's referendum has raised fears over corporate investment in Britain, which some economists fear could exacerbate difficult times ahead as the government negotiates future trade relations.

GSK's substantial investment therefore gives some reassurance and follows signs that foreign buyers, lured by a plunge in the pound, are looking to snap up bargains in Britain, led by Japanese group SoftBank's $32 billion swoop for chip designer ARM Holdings.

The board of French utility EDF, meanwhile, will meet on July 28 to consider a final investment decision on its $24 billion Hinkley Point C nuclear project in Britain.

The pharmaceuticals industry is a notable success story for Britain, directly employing more than 70,000 people and accounting for 25 percent of all business research and development spending.

RECOVERY PHASE

Witty has been under pressure since 2013 as profits have flagged and some investors have questioned his strategy, but he said he was "very positive" about handing over the company in a strong recovery phase when he retires next March.

GSK edged up its forecast for full-year core earnings per share (EPS) growth at constant currencies to 11 to 12 percent from 10 to 12 percent seen previously, after second-quarter core EPS of 24.5 pence soundly beat analyst forecasts of 21.0p.

Berenberg Bank analysts said the results were strong and the currency effect larger than expected, helping lift GSK shares by around 2 percent.

The currency tailwind, however, complicates accounting for GSK's majority-owned consumer health and HIV businesses, resulting in a 1.8 billion pounds charge. This reflects the fact that these operations are now worth more in sterling terms and GSK is therefore liable to pay its partners more if they decide to sell out.

GSK said its new manufacturing investments would create some new jobs but it did not give a number.

At Barnard Castle, it will spend 92 million pounds on a sterile facility to make biotech drugs, while Montrose will get 110 million for a new unit making lung drug ingredients, and 74 million at Ware will expand production of its Ellipta inhaler.

(Editing by David Holmes and Alexander Smith)