The rocketing price of cocoa has forced Mars to shrink the size of its Galaxy bars by 17%, less than a year after increasing the price 26%.

The take-home Galaxy bar is being shrunk from 150g to 125g to avoid the need for further price hikes such as last November's, when the shelf price rocketed from £1 to £1.26.

Cocoa is currently plagued by both supply issues caused by aging crops in the key West African growing region and by black pod disease, which renders over a third of each year's harvest unusable.

The cost price hikes probably made the move justifiable, said one industry expert, but warned that it was a risky move when consumers were generally expecting falling prices.

"Mars' tactic of reducing pack size less than a year after a substantial retail price hike is a dangerous game," she said. "Whether it was Mars' original intention, or just bad timing, this is a very raw deal for consumers, who are paying significantly more for significantly less.

"We know manufacturers use various methods to increase prices or decrease product sizes, often after a promotional period for example, so consumers are less aware. But these changes, so close together, could be a step too far."

Mars reduced the pack size of its sharing pouches earlier this year, but also reduced the price. A spokesman for the company said this was part of a range revamp that also saw the introduction of a larger family sharing bag. The Galaxy move, he said, was not a revamp, but instead related to cost increases shared by other industry players.

"Like all food manufacturers, we face significant cost increases due to rising commodity prices and higher production costs," he said. "Whilst we look to absorb these costs where we can, sometimes the weight or price of a product will change. We work hard to ensure that we continue to offer great value for money to our consumers"

Last year, rival Cadbury cut the size of its sharing bar by 20g, to 230g.