LONDON (Reuters) - The biggest shareholder in Sainsbury's ( JSNSF ), the Qatar Investment Authority (QIA), is selling 306 million pounds ($399 million) worth of shares in the British supermarket group.

According to regulatory filings, QIA offered 109.4 million shares at a price of 280 pence, about 5% of its holding.

Prior to the offer, QIA had a 14.2% stake in Sainsbury's ( JSNSF ), according to LSEG data.

Shares in Sainsbury's ( JSNSF ), which has an over 15% share of Britain's grocery market, trailing only Tesco, have risen 12% over the last year. They closed Thursday at 288 pence.

($1 = 0.7664 pounds)