North Korea, Somalia and South Sudan have been named the world's most corrupt nations amid warnings Britain and America could soon slip down the table.

Denmark was named the least corrupt country on the planet, followed by New Zealand and Finland, according to Transparency International.

The UK retained its 10th place ranking alongside Germany and Luxembourg but the group warned the country's reputation would take a hit if standards are weakened after Brexit.

The top 10 least corrupt countries also included Sweden, Switzerland, Norway, Singapore, the Netherlands and Canada. Australia remained 13th in the table.

But the US was placed 18th, down from 16th in 2015 with the research body warning there were worrying signs of populism in Donald Trump's presidential victory.

North Korea, Somalia and South Sudan have been named the world's most corrupt nations while Denmark was named least corrupt in Transparency International's table

South Sudan was named among the most corrupt nations on the planet according to new research (file picture)

Kim Jong-UN's North Korea was named among the world's most corrupt nations amid warnings Britain and America could soon slip down the list

At the bottom end of the scale, Somalia was ranked most corrupt, followed by South Sudan, North Korea and Syria.

The annual Corruption Perceptions Index gave the United Kingdom a ranking of 81, putting it in line with Germany and Luxembourg, and just steps behind the likes of New Zealand and Denmark, which topped the list with a score of 90.

But Transparency International warned that the UK could fall out of the top 10 if it fails to deliver a promised national Anti-Corruption Strategy, or weakens standards for short-term economic gains following Brexit.

UK executive director Robert Barrington said: 'Already, the uncertainty posed by Brexit has the potential to encourage a 'business at any cost' trade strategy; such an approach would be a disaster for UK's long-term reputation as a leading anti-corruption player.'

Barrington said a national Anti-Corruption strategy would help address growing disillusionment around politics and concerns about inequality - both of which fuelled the Brexit vote and propelled the rise of 'populist strong-man figures' abroad.

While the US was given a score of 74, Transparency International researcher Finn Heinrich warned: 'What we see particularly in the United States are the first signs of a person coming on board on a "drain the swamp" agenda (and) betraying that agenda already...with the nepotism and the appointments, with putting people in his cabinet who have conflicts of interests on various levels.

'So obviously it is too early to tell, but if you look at other populist leaders ... the track record of them doing what they promised is dismal.'

The report explained that voters who are 'fed up' with 'empty assurances' to tackle corruption are turning to populist politicians promising to change the system, but who end up introducing even worse regimes.

Autocratic and populist leaders often put democracy in jeopardy by cracking down on civil society, limiting press freedom and weakening independence of the courts, the organisation said.

It pointed to index scores for Hungary and Turkey - which have seen a rise of autocratic leaders Viktor Orban and Recep Tayyip Erdogan - drop to scores of 48 and 41, respectively.

But the US was placed 18th, down from 16th in 2015 with the research body warning there were worrying signs of populism in Donald Trump's presidential victory

The UK retained its 10th place ranking alongside Germany and Luxembourg but the group warned the country's reputation would take a hit if standards are weakened after Brexit

Meanwhile, Argentina - which ended 12 years of populist rule by electing centrist Mauricio Macri in 2015 - saw its score rise to 36 from 32.

The index scores countries on a range of factors, such as whether government officials are held to account or go unpunished for corruption, the perceived prevalence of bribery and whether public institutions respond to people's needs.

Nearly 70% of the 176 countries scored below 50 on the 100-point scale, with a zero meaning a country is perceived to be highly corrupt and 100 indicating it is perceived to be very clean.

'This year, more countries declined in the index than improved, showing the need for urgent action,' the report said.

The country that dropped most sharply in the rankings was Qatar, which has faced criticism over alleged human rights abuses involving migrant construction workers since it was chosen to host the 2022 World Cup.

At the bottom end of the scale, Somalia was ranked most corrupt, followed by South Sudan, North Korea and Syria (file picture)

Transparency International pointed to index scores for Hungary and Turkey - which have seen a rise of autocratic leaders Viktor Orban and Recep Tayyip Erdogan (pictured) - drop to scores of 48 and 41, respectively

It plunged 10 points, falling to 31st on the list from 22nd last year.

Still, Mr Heinrich said Qatar's government had in the past shown itself 'willing and keen' to fight corruption and Transparency would appeal for more fundamental reforms to ensure better freedom of speech and more media freedom, among other things.

'You can't fight corruption without having accountability and transparency in the entire public process,' he said.

Afghanistan, a perennial fixture near the bottom of the list, improved the most in 2016.

Its score on the Transparency International index rose four points, but was still ranked 169th, just ahead of Libya, Sudan and Yemen.

The organisation is now 'urgently' calling for 'deep-rooted systemic reforms' across the globe which address a growing imbalance of power and wealth, empowers citizens to stop widespread impunity for corruption, holds authorities to account and gives people a 'real say' in decisions affecting day-to-day life.

Transparency International says these reforms must include public registries that disclose company ownership, and penalties for professionals who help move corrupt money across borders.