Berkshire Hathaway, the holding company of US billionaire investor Warren Buffett, received a stunning $29 billion check last year from the US government, thanks to a new tax law that massively lowered corporate tax rates.

In his much-anticipated annual letter to shareholders, Buffett explained that the company's net gain of $65.3 billion in 2017 was only partly due to his employees' efforts.

"Only $36 billion came from Berkshire's operations," he wrote. "The remaining $29 billion was delivered to us in December when Congress rewrote the US Tax Code."

Still, Buffett assured stockholders, "the $65 billion gain is nonetheless real - rest assured of that."

The new law, greatly touted by President Donald Trump, lowered the tax rate paid by US corporations from 35 percent to 21 percent, allowing many to undertake major new outlays and others to book significant fiscal gains.

"Buffett has been quick to say that he thinks that the tax policy changes were going to help Berkshire and be positive overall," said Bill Smead, chief executive of Smead Capital Management in Seattle, a Berkshire shareholder. "It is a bit ironic his candidate would not have any interest in this whatsoever."