PepsiCo is making weather forecasters of Walkers crisp-munchers in a multimillion-pound campaign that will give consumers £10 when they correctly predict where it will rain in the UK and Ireland.

The Walkers Rainy Days campaign is PepsiCo's biggest of 2010 and Walkers' biggest push since last year's Do Us a Flavour activity. It will cash in on the British "obsession" with weather, according to PepsiCo, which has linked up with the Met Office and readied a multimillion-pound prize fund.

The initiative stars Gary Lineker and kicks off on 13 September, when promotional packs hit shelves and people can start forecasting on the Walkers website.

With each pack of Walkers crisps consumers get a "prediction point", which they can use to ­reserve one of 21,000 grid squares around the UK and Ireland.

Every day, the Walkers website will feature two dates and maps, on which people can make their prediction. The grid spots close either when all locations have been reserved or at 7.59am 24 hours before the day's map is in play.

Rainy Days account and bank codes can be set up between 13 September and 31 March 2011.

The forecast dates run from 1 October through to 17 November 2010, but a ­final forecast date of 2 April next year will be available for people who buy competition packs after the ­promotion has ended.

"Between October and November last year it rained almost 60% of the time," said marketing director Miranda Sambles. "At Walkers we rely on this wet weather to grow our 100% British potatoes, used to make Walkers crisps, but we know the nation hates it when it rains.

"To put a smile on their faces when it does rain, Walkers Rainy Days will be offering a substantial prize fund using a never-seen-before 'win and save' mechanic."

The campaign would tap into a national mood, as people are fed-up with yet another "non-barbecue weather" August, according to Kate Waddell, MD of consumer brands at Dragon Rouge.

"Walkers yet again seems to have tapped into the mood of the nation and is acting fast to gain attention and put a smile on summer-worn consumers' faces," she said.

"The limited edition pack designs bring some sunshine and stand-out to the shelf and put another layer of temptation in the way of consumers. Will they or won't they be able to predict rain? Surely easy money in the UK!"

Promotional packs will feature rainbow and raincloud motifs throughout the campaign.