Chancellor George Osborne is expected to announce further measures to help small businesses in an autumn budget later on Tuesday, on top of several billion pounds of loan guarantees already flagged by a source familiar with his plans.

The main measure will be a six-month extension of an existing exemption from municipal business taxation worth 210 million pounds, a Treasury source told Reuters.

Instead of expiring in October 2012, the small business rate relief scheme will continue until April 2013.

Osborne is under pressure to come up with measures to boost growth as he defends his flagship austerity programme against growing political criticism. New government forecasts look set to show little growth next year and a big overshoot in borrowing.

Other measures likely to be announced tomorrow include 50 percent income tax relief for individuals investing up to 100,000 pounds in company start-ups, as well as a one-year exemption on individuals' capital gains invested into start-ups. This will cost just under 100 million pounds over the three tax years starting 2013/14, the source said.

Osborne is also likely to announce that regional business development funds will be required to match angel investments of 0.2-2.0 million pounds into small and medium-sized businesses, up to a maximum of 50 million pounds, as well as speeding up payment of subcontractors on government construction projects.

On Sunday, a source close to the matter told Reuters that Osborne will next week unveil schemes worth at least 10 billion pounds to boost credit to small and medium sized firms, including a government guarantee for bank loans, and plans to co-invest in a new fund to lend to larger firms.

(Reporting by David Milliken; editing by Ron Askew)