Legendary investor Warren Buffett lost nearly a billion dollars after he failed consult his right hand man and long term business partner Charlie Munger.

In Buffett's annual letter from his company Berkshire Hathaway, which was released at the weekend, he revealed that he made "a big mistake" by not consulting Munger before ploughing $2bn (£1.2bn, €1.5bn) into buying Energy Future Holdings (EHF).

"Unless natural gas prices soar, EHF will almost certainly file for bankruptcy in 2014," said Buffett.

"Last year, we sold our holdings for $259m. While owning the bonds, we received $837m in cash interest.

"Overall, therefore, we suffered a pre-tax loss of $873m. Next time I'll call Charlie [Munger].

"Charlie Munger and I believe both Berkshire's book value and intrinsic value will outperform the S&P in years when the market is down or moderately up," said Buffett.

"We expect to fall short, though, in years when the market is strong – as we did in 2013.

"We have underperformed in 10 of our 49 years, with all but one of our shortfalls occurring when the S&P gain exceeded 15%."

Despite the EHF loss, Buffet made the most money in 2013.

The chairman of Berkshire, who is consistently ranked among the world's wealthiest people, made about $37m per day in 2013 to become the biggest financial gainer in 2013, according to research firm Wealth-X, which focuses on high net worth individuals.

Buffet's net worth was $59.1bn at the end of 2013, up $12.7bn from $46.4bn at the beginning of that year.