A rampant Tesco is piling pressure on its rivals with ever lower prices that other supermarkets struggle to match.

An Evening Standard survey has found that Britain's biggest grocery chain undercuts even relatively downmarket Asda across a wide range of lines.

Retail experts say the awesome buying power of the £30 billion group, combined with "breathtakingly" detailed surveillance of its rivals' prices, is giving Tesco a crucial edge with consumers.

The company, headed by chief executive Sir Terry Leahy, delighted the City last week with a 24 per cent leap in first-half profits to £822 million, far higher than forecast. Group pre-tax profits are expected to beat £2 billion for the financial year as a whole.

Once mocked as "a poor woman's Sainsbury's", Tesco is now grabbing market share from its grocery rivals and is said to account for one pound in every eight spent in British shops.

Tesco's razor-sharp pricing is particularly noticeable on everyday staples such as white bread (19p), tea bags (80 for 31p) and sugar (68p for 1kg).

With the exception of Waitrose, all the supermarkets are branching out into new territory - clothes. Asda has the popular George range, while Tesco is gaining ground with the catwalk-inspired Florence & Fred.

Clothing is seen as having huge potential for food stores. At the end of the summer holidays, Tesco slashed 2.5million off its range of school uniform, and Asda and M&S followed. A man's T-shirt in Tesco costs £1.75 compared with ?8 in M&S. In our survey, a typical food basket cost £3.27 at Tesco, 25 per cent below the £4.38 charged at the next cheapest, Sainsbury's. The Asda bill was the next lowest at £5.24, with Waitrose on £5.66 and Marks & Spencer dearest at £6.36.

Adding wine and "treats" such as Parma ham and fruit tarts to the basket closed the gap a little. However, the Tesco bill still came out lowest at £15.52, some 13 per cent below Asda on £17.87.

Retail specialists said Tesco was reaping the benefit of 15 years of heavy investment in sophisticated systems for monitoring its rivals' prices. These are "light years ahead of anything else in the UK and in all honesty they are world class," according to supermarket expert Andrew Seth.

The system uses state-of-the-art IT to analyse thousands of prices at nearby rivals on a daily or weekly basis, and adjusts its own prices accordingly. Even major US supermarket groups model their versions on the Tesco template.

Mr Seth said Tesco employed hundreds of computer analysts to "crunch" the vast amounts of incoming pricing data in the biggest IT department of its kind in Britain. The company is also known for its mercilessly tough negotiations with suppliers, forcing them to bid against each other.

However, critics say Tesco's apparently low prices come at a huge social cost. Jessica Bridges Palmer, of the New Economics Foundation think tank, said: "There's no such thing as cheap food. You might be paying a low price at the till but someone is paying the full price somewhere." She said bargain food was a delusion because it was often full of chemicals or lacking in nutrients.