The United States has dropped out of the top 20 list of “cleanest countries”, according to an annual survey that ranks countries based on corruption.

The US saw its ranking on the closely watched annual survey drop as a result of threats to its constitutional system of checks and balances. Overall, the US dropped to a score of 71, from 75 in 2017.

The ranking means that the US dropped out of the top 20 nations on the Corruption Perceptions Index (CPI) for the first time since 2011.

“A four point drop in the CPI score is a red flag and comes at a time when the US is experiencing threats to its system of checks and balance, as well as an erosion of ethical norms at the highest levels of power,” the organisation, which is based in Berlin, said in a statement.

“If this trend continues, it would indicate a serious corruption problem in a country that has taken a lead on the issue globally — this is a bipartisan issue that requires a bipartisan solution”, the statement said.

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The watchdog group ranks countries on a scale of zero to 100, with the higher number indicating a very clean government.

All told, more than two-thirds of the countries in the survey scored below 50 points.

A cross-analysis of the survey data with global democracy data found a correlation between perceived strong and transparent democracies, and their corruption rankings.

Full democracies in global democracy data, for instance, scored an average of 75 on the CPI. Flawed democracies averaged 49. Autocratic regimes, meanwhile, averaged 30 points.

The report cited Freedom House’s annual democracy survey, and said that Turkey was downgraded from “partly free” to “not free”. Hungary dropped to its lowest score since 1989.

The ratings reflect the “deterioration of rule of law and democratic institutions, as well as a rapidly shrinking space for civil society and independent media,” Freedom House said in a statement.

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“Our research makes a clear link between having a healthy democracy and successfully fighting public sector corruption,” Delia Ferreira Rubio, the head of transparency for the organization, said. “Corruption is much more likely to flourish where democratic foundations are weak and, as we have seen in many countries, where undemocratic and populist politicians can use it to their advantage”.

Denmark led the survey as the least corrupt nation, with a score of 88. New Zealand, Finalnd, Singapore, Sweden, and Switzerland followed close behind. The others at the top of the list were Norway, Netherlands, Canada, Luxembourg, Germany, and Great Britain.

Somalia was the most corrupt with 10 points, followed by Syria, Sout Sudan, Yemen, North Korea, Sudan, Guinea Bissau, Equatorial Guinea, Afghanistan, and Libya.