The Economist Intelligence Unit (EIU) produces an annual review ranking countries on their adherence to 60 distinct democratic values, including electoral processes and press freedom.

The US scored of 7.98 out of 10 last year, dropping into the flawed democracy category for the first time. The change reflected what the report said was “a sharp fall in popular confidence in the functioning of public institutions”. This predated and aided the election of Donald Trump, according to the think-tank.

The US scored 7.98 again in 2017, placing it behind 19 full democracies including Norway, Ireland, the UK, Uruguay and Spain.

Now ranking 21st, the US was among other countries with flawed democratic systems including Italy, South Korea, France, Chile, Botswana and Mexico.

It was at risk of sliding still further towards authoritarianism, the report claimed.

“If Mr Trump is unable to reverse the trend towards increasing social polarisation, US democracy will be at greater risk of further deterioration, especially given the interplay of this trend with other, long-standing drivers of democratic decline. For example, the US scores comparatively poorly in the democracy index in the functioning of government category,” the report said.

The manner in which US congressional districts had been redrawn in recent years had polarised the political landscape, it claimed.

“As a result, members of Congress fear a challenge in their party primaries, which are controlled by the party base, and are consequently incentivised to move to the right (for Republicans) or to the left (for Democrats).

“The upshot is a stronger emphasis on ideological purity and less appetite for compromise, which reinforces existing cynicism among voters about the workings of Congress.”

Meanwhile, major political events such as the Iraq wars, the financial crisis of 2008/09 and repeated federal government shutdowns had had eroded confidence in government over decades, according to the report.

A growing wealth divide was further fuelling public dissatisfaction with the government, the report said.

The EIU, which is part of the same company as The Economist magazine, predicted Mr Trump’s tax bill, which favours those on high incomes, would further fuel income inequality.

The US’ poor democracy rating comes amid a global regression of democratic principles, the report said, with the average score sliding from 5.52 in 2016 to 5.48 last year.

Some 89 countries experienced a decline in their total score compared with 2016, more than three times as many as the countries that recorded an improvement.

This was the worst performance since 2010/11 in the aftermath of the global financial crash.