The proposal makes the Lib Dems the first major party to take up the idea, long touted by food campaigners, of using taxes to promote healthy diets.

Cable told the party's conference in Liverpool that VAT on "healthy" drinks should be cut from 17.5% to 5%.

It follows signatures from 20,000 people on an online petition posted on the Downing Street website by Innocent, which makes juices and smoothies. It put forward the idea that an extra 500 million more portions of fruit would be consumed every year if VAT on fruit drinks was abolished.

Such ideas are controversial and await scientific validation. Critics point out, for example, that fruit juices and smoothies contain very high levels of sugar.

A single 500ml bottle of fruit juice can contain the equivalent of 8-10 teaspoons of sugar. Smoothies can be even more calorific as they are often mixed with yoghourt. It could meant that cutting taxes on such drinks would promote obesity.

Cable's announcement comes a day after the Tories said they would hike taxes on super-strength beer, cider and alcopops, in an attempt to target binge drinking. The Government is also widely expected to increase tax on alcohol in next week's budget.

Cable batted off suggestions he was stealing policies from political rivals saying: "If all the three parties should be saying something not greatly dissimilar then that may be a new form of grown-up politics.

"It's very odd that some taxes that are regressive and very unpopular - like council tax - have run way ahead of inflation, while the duty on drinks - particularly on spirits and wine - has lagged behind."

Under current tax rules fruit juices attract the full rate of 17.5% VAT while many supposedly "unhealthy" foods such as frozen pizza are zero-rated.

However, the price of smoothies, even factoring in a VAT reduction, is considerably higher than the price of most fruit and vegetables, which are already VAT free.

NHS guidelines stipulate that either a glass of 100% juice (fruit or vegetable) or a 150 ml fruit smoothie only count as one portion of the recommended five portions of fruit and vegetable you should consume each day.

Innocent states on the Downing Street website: "It's a simple step, and, as more and more people get their fruit in liquid form, it could make a big difference."

A spokeswoman for the British Nutrition Foundation said: "We welcome anything that reduces the cost of healthy food so that people can improve their nutrition."

A Treasury spokesman warned that such policies could prove hard to implement. He said: "The 2004 Wanless report on public health highlighted the difficulties of using the tax system to promote public health."