Sainsbury's has launched a stinging attack of Tesco's £4m 'swingometer' price comparison campaign, describing it as "inaccurate" and misleading.

During the campaign, which ran from July to September, Tesco boasted that 4,251 products cost less at Tesco than at Sainsbury's, while only 470 cost more.

However, in the latest issue of Sainsbury's Journal, head of price and promotion, Karen Kirner, accused Tesco of sharp practice and insisted Sainsbury's price checked thousands of items each week to ensure they matched Tesco's.

"When the Tesco swingometer came out I was furious," said Kirner. "I couldn't believe they were trying to convince everyone they were cheaper than us, not when my team and I spend five days a week making sure that they're not. Those figures were inaccurate to say the least."

Buyers were expected to review any higher prices and "make them competitive again", she said. The only exception was where a difference in quality justified a higher price. "The message we want to get out is great products at fair prices," she said. "That's something Tesco can't compete with. "

Her comments were echoed by another industry source who described Tesco's price comparisons as "wildly inaccurate".

"I don't believe they can be particularly reliable," he said. "It may be able to omit certain lines from the comparison, if the competitor's price is cheaper. There also seems to be a time lag between a competitor dropping a price and then Tesco recording the change."

However, Tesco dismissed the criticism and challenged Sainsbury's to be more transparent about its prices.

"Our price checker is accurate. We're the only supermarket with the confidence to publish our prices online along with those of our competitors so customers can judge for themselves," said a spokesman.

"We challenge Sainsbury's to do the same, which would show their customers how much more expensive they are."