LONDON (Reuters) - J Sainsbury, Britain's third-biggest supermarkets operator, said on Monday that it is to enter the mobile phone service market later this year as it seeks to add more business streams to complement its core grocery offer.

The firm, which trails Tesco and Wal-Mart's Asda by annual revenue, said it has teamed up with Vodafone to create the "Mobile by Sainsbury's" network service.

Details of the joint venture's product range and pricing will be announced at launch later this summer.

Sainsbury's currently sells mobile phones and offers prepay SIM cards for other networks but does not provide its own virtual network service. Tesco has been offering mobile services since 2003.

Going into new business markets forms a core part of Sainsbury's strategy. Over the last year, it has entered the markets for eBooks, mp3 downloads and video on demand services.

In May, the firm bought out the 50 per cent of the Sainsbury's Bank joint venture with Lloyds Banking Group it did not own.

Last month, Sainsbury's outperformed Tesco with a rise in first-quarter like-for-like sales.

Shares in the firm were down 0.3 pence at 355.2 pence at 0819 GMT, valuing the business at 6.75 billion pounds (S$13 billion).