UPDATE 1-GW expects UK to approve cannabis drug by end-June

* UK regulatory approval of Sativex seen before end of Q2

* Spanish green light expected shortly thereafter

* Marketing partners Bayer and Almirall preparing for launch

* H1 net loss pretax 2.7 mln stg vs year-ago 4.0 mln profit

(Adds detail on Sativex, background)

LONDON, May 20 (Reuters) - GW Pharmaceuticals GWP.L expects its groundbreaking -- but much delayed -- cannabis-based medicine for multiple sclerosis to be approved in Britain before the end of June.

A regulatory green light in Spain is likely shortly thereafter, the British-based drugmaker said on Thursday, adding that regulators in both countries had agreed that all major and minor issues related to the application had been resolved.

The medicine, which is sprayed under the tongue, is to be marketed in Britain by Germany's Bayer BAYGn.DE and in the rest of Europe by Spain's Almirall ALM.MC.

Both marketing partners are now well advanced in preparations for the Sativex launch, GW said.

GW will get a 10 million pounds ($14.35 million) milestone payment from Bayer on British approval, while a further 2.5 million pounds is payable by Almirall following both regulatory and pricing approval in Spain.

Clinical trials have shown GW’s drug Sativex reduces spasticity in multiple sclerosis patients who do not respond adequately to existing therapies.

It became the world’s first cannabis medicine to win regulatory approval when it was approved in Canada in 2005. Its roll-out in Britain, Spain and other European markets thereafter represents a much larger commercial opportunity.

GW reported a net loss before tax for the half year to March 31 of 2.7 million pounds, against a profit of 4.0 million in the year-ago period and a loss of 2.9 million in the six months to Sept. 30, 2009. (Reporting by Ben Hirschler)