Image copyright CQ Roll Call via Getty Images Image caption Michigan Congressman Justin Amash is now sitting as an independent

US congressman Justin Amash, a fierce critic of President Donald Trump, has quit the Republican Party.

He announced his decision to sit as an independent in the Washington Post, issuing a warning about political hyper-partisanship in Washington.

"Modern politics is trapped in a partisan death spiral," the Michigan representative wrote on Thursday.

Mr Amash was the first Republican congressman to call for Mr Trump's impeachment.

His reasoning for leaving the Republicans was his overall disenchantment with party politics, he said.

"The two-party system has evolved into an existential threat to American principles and institutions," the five-term congressman said, in the op-ed published on the 4 July Independence Day holiday.

Mr Amash said he believed most Americans are not "rigidly partisan and do not feel well represented by either of the two major parties".

There might also have been some political calculation, says BBC North America reporter Anthony Zurcher.

Polls in his district suggest he would have an uphill battle to win a Republican primary contest ahead of 2020 elections, he adds.

President Trump reacted on Twitter to Mr Amash's decision, calling it "great news for the Republican Party".

Mr Amash had previously butted heads with the US president.

In February, he became the sole Republican to co-sponsor a resolution in the US House of Representatives to reject the emergency Mr Trump declared at the US-Mexico border in order to build a wall there.

In May, he accused Attorney General William Barr of misleading the public about Special Counsel Robert Mueller's findings on Russian interference.

He claimed Mr Trump had "engaged in specific actions" that "meet the threshold for impeachment".

Media playback is unsupported on your device Media caption The Mueller report - in 60 seconds

Mr Mueller's report found insufficient evidence of a criminal conspiracy between Trump's 2016 election campaign and Russia to influence the outcome.

However it did find 10 instances where Mr Trump possibly attempted to impede the investigation.

Mr Amash ended his op-ed in the Washington Post with an appeal to Americans to join him "in rejecting the partisan loyalties and rhetoric that divide and dehumanize us".

"I'm asking you to believe that we can do better than this two-party system — and to work toward it. If we continue to take America for granted, we will lose it."