* Union says preparing for a strike
* Strike could involve 18,000 workers
* Lonmin shares in London up 0.8 percent, lag sector
(Adds company comment, shares)
JOHANNESBURG, Oct 28 (Reuters) - South Africa's National Union of Mineworkers said on Thursday wage talks with platinum producer Lonmin had collapsed, which could lead to a strike by 18,000 of its members.
The union said it was demanding a wage rise of 11 percent while the company was offering 7 percent.
"We are already preparing ground for a strike against Lonmin for we no longer have a choice," said NUM`s chief negotiator at Lonmin, Erick Gcilitshana.
A spokesman for Lonmin in London said the company was following "due process", but declined further comment.
Under South African labour law, when negotiations reach a deadlock, the two sides must bring in mediators before a strike can be called.
Lonmin shares in London rose 0.8 percent to 1750 pence by 1007 GMT, lagging a 1.2 percent rise in the British mining index.
South Africa, the continent's biggest economy, has been hit by a wave of strikes and strike threats in both the private and public sectors, which have led to above-inflation settlements and stoked fears that the cost of living will rise.
The inflation rate slowed to a five-year low of 3.2 percent in September. (Reporting by Shapi Shacinda; Additional reporting by Eric Onstad in London; Editing by Matthew Tostevin and Michael Shields)