According to a regulatory filing with the US Securities and Exchange Commission, Berkshire now owns 57.98 million shares in the company, or more than 10.8% of the stock.

In February 2014, Berkshire Hathaway traded two-thirds of its shares in the Houston-based oil refiner, swapping $1.35bn of shares for a chemicals business.

Phillips has now more than doubled in value along with a number of other refiners in the country.

Lipow Oil Associates president Andy Lipow was quoted by Bloomberg as saying: "Lower oil prices are precipitating an upwards revised forecast for world demand.

"Refiners are going to see decent margins going forward."

However, the latest filling didn’t mention whether it was Buffett or his deputy investment managers who built the recent stake.

In February this year, Berkshire Hathaway divested its stake in Exxon Mobil for $3.74bn, amid falling oil prices.

The company bought a new 24.7 million share stake in Deere & Co. (DE), an agricultural equipment manufacturer.

The 41 million-share stake in Exxon Mobil was acquired by Berkshire in 2013, when the oil price was at its peak.