LONDON, April 19 (Reuters) - Canada became the first country in the world to approve a cannabis-derived medicine on Tuesday when it gave a green light to a mouth spray developed by Britain's GW Pharmaceuticals Plc .

The go-ahead had been expected after Canadian regulators said last December that Sativex, which is sprayed under the tongue, qualified to be considered for approval.

But the news still boosted GW shares as much as 14 percent, reflecting the importance of the product to the small biotechnology firm.

Sativex, which is designed to help patients with multiple sclerosis (MS), will be marketed in Canada by Germany's Bayer AG and is likely to launched in May, according to GW Pharma Executive Chairman Geoffrey Guy.

Guy has long championed the benefits of certain chemicals in cannabis for countering the neuropathic, or nerve, pain suffered by many MS patients.

But his company's product has suffered a series of delays and setbacks in development, which have sent its shares on a rollercoaster ride.

The company had originally hoped to win British approval for Sativex in 2003 but UK officials said in December they wanted more evidence about the benefits of the medicine. It is not expected to be approved in its home market until later this year or possibly 2006.

GLOBAL AMBITIONS

Nevertheless, Guy said he was confident the drug would win approval in major markets around the world, underpinning its "significant" sales potential.

"Canada is the first approval of a global regulatory strategy which will probably take us about five years, after which we should be in virtually all major countries, except perhaps Japan," he told Reuters.

He declined to forecast sales for the product and said the price would only be announced by Bayer shortly before launch.

GW Pharma grows thousands of marijuana plants at a secret location in the English countryside, having been granted a dispensation by the government to use the plant for medical research.

Cannabis has a long history of medicinal use, dating back to ancient Chinese times. Queen Victoria, whose physician described it as "one of the most valuable medicines we possess", is said to have taken cannabis tincture for her menstrual pains.

But it fell out of favour in the 20th century because of a lack of standardised preparations and the development of more potent synthetic painkillers.

GW Pharma believes it has got round those standardisation problems with its spray-based product, which also avoids the damaging effects of smoking the drug.

Its approval was welcomed by the MS Society of Canada, whose national medical adviser Dr William McIlroy said sufferers needed new options to address their pain.

Shares in GW Pharma -- which were floated at 182 pence each in 2001 -- jumped to a four-month high of 138-1/2p following the Canadian green light before paring some of the gain to trade up 11 percent at 134-1/2p by 1215 GMT.