AFP - British supermarket chain Sainsbury said on Wednesday that net profits leapt almost 50 percent in the first half of its financial year, lifted by lower costs and sales growth.

However, the group also warned that the trading environment would remain challenging for the rest of its 2009/2010 fiscal year a fierce recession in Britain.

Net profits surged 48.2 percent to 252 million pounds (280 million euros, 422 million dollars) in the 28 weeks to October 3, compared with the equivalent period of the previous year, Sainsbury said in a results statement.

Total sales climbed 3.7 percent to 11.158 billion pounds during the reporting period. Profits were also boosted by the revaluation of properties in the group's joint ventures.

"Tight control on operating costs is enabling further investment in the customer offer (of products) and, together with our good sales performance, has delivered further strong profit growth," said Chief Executive Justin King.

But he added: "As we enter the second half we expect the economic environment to remain challenging and market growth to slow due to reduced food price inflation."

Underlying profits before tax, meanwhile, rose 18.5 percent to 307 million pounds in the first half.

That was broadly in line with market expectations of 300 million pounds, according to analysts polled by Dow Jones Newswires.

In response, Sainsbury shares rose 3.66 percent to 339.6 pence in morning trade on London's FTSE 100 index of leading shares, which was 0.79 percent higher at 5,271.72 points.

"The numbers were towards the top end of expectations, but were immediately overshadowed by a cautious company outlook for the remainder of the year," said analyst Richard Hunter at Hargreaves Lansdown Stockbrokers.

Sainsbury's is Britain's third biggest supermarket chain after Wall-Mart owned Asda and the country's largest retailer Tesco.