The Rainy Days promotion offered the chance to win £10 if entrants could correctly forecast where and when in the country it would rain.
A complainant said that they had bought enough packets of crisps to enter the promotion twice a day but the company later stated that entries had been reduced to one per day per person.
Walkers, owned by PepsiCo, said the original allowance of two plays per day was based on demand for previous promotions but added that it believed the mechanic of Rainy Days had not been used before.
It said that during the “initial launch phase” it became clear demand from customers wanting to play far outstripped supply of the available grid spots for the game – these were limited to 21,271 different areas per day. Therefore the number of plays per day was finite and coupled with the high level of demand, Walkers decided it had to be fair to the wider customer base who wanted to play.
The decision to restrict entries to one per person each day was made before marketing began. The company also said that the advertising and crisp packs did not state the number of plays permissible per day but advised customers to check the terms and conditions online, which had been updated.
Customers who registered an account during the launch phase were contacted by email about the change to T&Cs.
The Advertising Standards Authority (ASA) said that more than 40,000 people had registered and taken part in the promotion over seven weeks and that they would expect to be able to purchase and bank the promotional codes in accordance with the orginal T&Cs.
It ruled that it was not acceptable for Walkers to alter the rules of play during the competition and the promotion must not run again in this form.