Bernie Sanders’ became a millionaire in 2016, putting him in the top 1 per cent of earners in the US, according to the long-anticipated release of the senator’s tax returns.

The release confirms Mr Sanders’ income crossed the $1m (£764,000) threshold in 2016 and 2017, although he reported less earnings in his most recent return.

In the Vermont senator’s 2018 return, he and his wife, Jane Sanders, earned more than $550,000, including $133,000 income from his Senate salary and $391,000 in books sales.

The Sanders campaign said he paid 26 per cent effective tax rate in 2018.

The release comes as Mr Sanders faces criticism that his newfound wealth contradicts his political message of democratic socialism in his 2020 presidential campaign.

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During a Fox News town hall meeting on Monday, the senator said his increased income came from the publication of his book, Our Revolution: A Future to Believe In.

At the televised meeting, Mr Sanders said he would not apologise for his recent wealth.

“That money in my case, and my wife’s case, came from a book I wrote, a pretty good book, you might want to read it,” he said.

“It was a bestseller, it sold all over the world, and we made money. So if anyone thinks I should apologise for writing a bestselling book, I’m sorry I’m not going to do it.”

He also challenged Donald Trump to release his tax returns.

“I guess the president watches your network a little bit, right?” Mr Sanders said to moderators Bret Baier and Martha MacCallum. “Hey, President Trump. My wife and I just released 10 years. Please do the same.”

Mr Trump broke from decades of presidential tradition by refusing to show voters his tax returns during his 2016 presidential campaign.

In 2016, Mr Sanders released just one year of his tax returns and faced criticism from his Democratic rival Hillary Clinton for refusing to release multiple years of tax documents.

Mr Sanders’ tax returns show he is among the top 1 per cent of earners in the US, which some critics have argued undermines his calls for an economy and government that works for everyone, not just the 1 per cent.

According to the Economic Policy Institute, families in the US earning $421,926 or more a year are part of this group.

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“These tax returns show that our family has been fortunate,” the senator said in a statement. “I am very grateful for that, as I grew up in a family that lived paycheck to paycheck and I know the stress of economic insecurity.”

“I will continue to fight to make our tax system more progressive so that our country has the resources to guarantee the American dream to all people.”

The release also disclosed $36,300 of charitable contributions by Mr Sanders and his wife in 2017, although the return does not detail their individual contributions.

In 2017, the couple announced that they had donated $25,000 as a grant to launch the Sanders Institute, a non-profit educational organisation aligned with the senator’s political and ideological beliefs.

Jane Sanders was a co-founder of the non-profit, along with her son, David Driscoll. However, the institute was put on hiatus last month amid criticism that the organisation blurred lines between family, fundraising and campaigning. Ms Sanders said the non-profit would cease operations beginning in May “so there could not even be an appearance of impropriety”.

The release of Mr Sanders’ returns follows a number of Democratic presidential candidates who have released tax records, including senators Kamala Harris, Elizabeth Warren, Kirsten Gillibrand and Amy Klobuchar.

Fellow candidate Beto O’Rourke also recently released a decade of tax records, which showed he and his wife paid $81,000 in taxes on an income of $366,000 in 2017.