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Prince Charles slams supermarkets as he turns 65

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By the time most people hit 65, they are thinking of giving up work and preparing for a life of leisure. But what if you've yet to start the job you were meant to do?

The heir to the British throne, Prince Charles, marked his 65th birthday by slamming the U.K.'s big supermarket chains for squeezing farmers' incomes by taking "none of the risk."

To mark his birthday, the future king guest-edited Country Life, the 116-year-old weekly magazine for the U.K.'s landed gentry, and wrote an impassioned editorial about Britain's struggling farming industry, which he said was facing some of its "toughest challenges."

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"Small farmers find themselves in the iniquitous position of taking the biggest risk, often acting as the buffer for the retailer and consumer against all the economic uncertainties of producing food, but receiving the least return," he wrote on Wednesday.

"It cannot be right that a typical hill farmer earns just £12,600, with some surviving on as little as £8,000 a year, whilst the big retailers and their shareholders do so much better out of the deal, having taken none of the risk."

The comments were the prince's most outspoken criticism of the U.K.'s supermarkets to date. Britain's food retail is dominated by the four big players - Tesco (the U.K.'s largest retailer by sales), Asda, Sainsbury's and Morrisons. The British Retail Consortium, which represents these supermarkets, said retailers were confident their investment in the food chain meant a strong future for farmers, and that a "high proportion" of supermarket food would continue to be sources in the U.K.

The prince said the current system squeezes farm incomes, meaning that many small and medium-sized farms could not afford to make crucial long-term reinvestment.

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"I fear this will create huge problems in the near future, especially in the dairy sector," he added.

Agriculture accounts for 0.7 percent of the U.K.'s gross domestic product, contributing around £26 billion a year to the economy, according to the Office for National Statistics. Output in the sector increased by 1.4 percent in the third quarter of 2013 from the second, but fell 1.6 percent when compared with the same period in 2012.

Prince Charles is a large landowner has long been a supporter of protecting Britain's countryside, which he described in Country Life as "the unacknowledged backbone of our national identity." His private estate, the Duchy of Cornwall, consists of over 53,000 hectares of land and includes an organic farm.

He is also known to weigh in on other issues, including the environment, architecture and alternative medicine. Last month the prince raised concerns about the U.K.'s pension system, arguing that fund managers renounce their short-sighted obsession with "quarterly capitalism" and invest in companies that tackle environmental and social challenges.

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Prince Charles is also no stranger to controversy in his private life, after splitting with Diana, Princess of Wales in the 1990s.

He married Camilla Parker Bowles in 2005 and took the opportunity as guest editor of Country Life to feature a picture of Camilla on the magazine's infamous frontispiece. The popular "girl in pearls" page has been a feature of the magazine since its launch in 1897.

Prince Charles has been heir to the British throne since he was three, and at 65 will be the oldest heir to the throne for almost 300 years. He will donate his pension to a charity which supports the elderly.

By CNBC's Katrina Bishop. Follow her on Twitter @KatrinaBishop and Google