LONDON (Reuters) - Part-nationalised Royal Bank of Scotland has started the sale process for over 300 of its British branches, people familiar with the matter said on Sunday.

RBS has sent out a teaser document to interested parties and appointed UBS to handle the process, the sources said.

Potential bidders have been given until the end of the month to express an interest, one source said.

RBS is being forced to sell RBS-branded branches in England and Wales and NatWest branches in Scotland by the Europe Union's competition body for receiving government support, but has been given up to four years to do so.

One banking source not connected to the deal said National Australia Bank and Spanish bank Santander could be interested.

RBS sold part of its asset management business last week and is close to selling its majority stake in its RBS Sempra commodities joint venture.

RBS declined to comment. UBS could not be reached.

(Reporting by Victoria Howley, Clara Ferreira-Marques and Steve Slater; Editing by Dan Lalor)