Lottery operator Camelot has completed an upgrade of all its terminals following a blunder which allowed stores to process out-of-date play slips for the new-format Thunderball game thus diminishing customers' chances of winning the jackpot.

The game's format was changed in May when five additional numbers were added, and new slips were distributed to all 28,500 stores with National Lottery terminals.

However, terminals were not updated before the launch of the new game, meaning that machines could still process the old 34-number play slips.

The error meant that the small number of customer who purchased tickets using the old slips had a diminished chance of winning the jackpot, or in some cases none at all, given that one of the five new numbers has been drawn in 27 games since the relaunch.

Camelot insisted that only a small number of retailers had made the mistake and said that terminals had not been upgraded prior to the relaunch because it wanted to minimise player disruption.

The company added that retailers should also expect to see more store visits by the Camelot sales force in the coming weeks to ensure that the new slips were being used.

Sarah Tarrant of The Corner Shop in Bournemouth, Dorset, said she was surprised stores weren't aware they were using the wrong playslips. "Camelot couldn't have done any more to let us know that the game was changing. We got messages on terminals every morning and notes in the retailer newsletter," said Sarah. "I can't imagine how stores could have missed this."

A Camelot spokeswoman advised any retailer who is asked by customers for a refund to give them the player hotline number so that they can contact the company directly.

Topics