A major till breakdown in more than 100 Tesco stores on Monday cost the retail giant as much as £5m, experts have estimated.

The breakdown, which coincided with the rollout of the new Clubcard system, forced 65 of the retailer's larger stores to close for up to four hours.

Insiders told The Grocer it was caused by an individual from head office "pressing the wrong button" while carrying out an IT update.

The update, described by Tesco as "routine", was probably sent before it had been tested on Sunday night, an IT expert said.

Another source close to Tesco said: "It was down to their new loyalty scheme. They have only themselves to blame."

Only stores with a particular IT configuration were affected by the breakdown, but it would have cost Tesco £2m-£5m in lost sales, said another source.

However, consumer analyst Dave McCarthy of Hobart Capital said the figure was negligible in the greater scheme of things.

"For a business that turns over more than £50bn a year, losing a couple of million because of a hiccup in the system is neither here nor there, but was an embarrassment just as Clubcard was relaunched," he said.

Tesco did its best to play down the incident and denied any connection with the highly publicised £150m relaunch of Clubcard and its new ad campaign featuring actors Fay Ripley and Mark Addy.

Meanwhile, Asda has claimed it attracted an average 10% more customers on Monday morning in stores close to affected Tesco branches.