Keir Starmer joined European allies warning of a 'dangerous downward spiral' in Nato today after Donald Trump threatened a trade war over Greenland.

A joint statement from Denmark, Finland, France, Germany, the Netherlands, Norway, Sweden and the UK said they stood 'firmly behind' the 'principles of sovereignty and territorial integrity'.

The president has shocked the military alliance by warning that opponents of an American takeover of the Danish territory will be hit with punitive tariffs from February 1.

In a bombshell post on his social media site, Mr Trump said levies will start at 10 per cent - and potentially rise to 25 per cent if they have not capitulated by June. 

But in a joint response this afternoon, the European powers said: 'As members of Nato, we are committed to strengthening Arctic security as a shared transatlantic interest. The pre-coordinated Danish exercise Arctic Endurance conducted with Allies, responds to this necessity. It poses no threat to anyone.

'We stand in full solidarity with the Kingdom of Denmark and the people of Greenland. Building on the process begun last week, we stand ready to engage in a dialogue based on the principles of sovereignty and territorial integrity that we stand firmly behind.

'Tariff threats undermine transatlantic relations and risk a dangerous downward spiral. We will continue to stand united and coordinated in our response. We are committed to upholding our sovereignty.'

Mr Trump's intervention has sparked fresh fears he is intent on ripping apart the Western pact that has maintained world peace for the past eight decades.

MPs also joined the backlash, with calls for the mooted trip by King Charles to Washington in the Spring to be cancelled. 'The civilised world can deal with Trump no longer. He is a gangster pirate,' senior Tory Simon Hoare said. 

Running the gauntlet of broadcast studios for the government this morning, Culture Secretary Lisa Nandy stressed that support for Greenland's sovereignty was 'non-negotiable'.

'We believe it is wrong, we believe it is deeply unhelpful,' she told the BBC's Sunday with Laura Kuenssberg of Mr Trump's tariff threat. 

But pressed on the issue of the state visit she merely referred to the 'depth' of the Transatlantic relationship. 

Economists have raised concerns that the UK will be back on 'recession watch' if the US goes ahead with tariffs. 

Senior MEPs have warned that the EU-US trade deal will be frozen in reprisals, with the spectre of a full-blown global trade war again looming. 

Sir Keir delivered some of his strongest criticism of Mr Trump last night, having expended huge effort trying to maintain warm ties with the president.

The PM said: 'Our position on Greenland is very clear – it is part of the Kingdom of Denmark and its future is a matter for the Greenlanders and the Danes.

'We have also made clear that Arctic security matters for the whole of Nato, and allies should all do more together to address the threat from Russia across different parts of the Arctic.

'Applying tariffs on allies for pursuing the collective security of Nato allies is completely wrong. We will, of course, be pursuing this directly with the US administration.'

Emmanuel Macron also reacted furiously to the news, promising consultation with EU leaders. The French president said: 'No intimidation nor threat will influence us, neither in Ukraine, nor in Greenland, nor anywhere else in the world when we are confronted with such situations.

'Tariff threats are unacceptable and have no place in this context. Europeans will respond to them in a united and co-ordinated manner if they were to be confirmed. We know how to uphold European sovereignty.'

Nigel Farage gave a more limited rebuke to the US president, posting on X: 'We don't always agree with the US government and in this case we certainly don't. These tariffs will hurt us.' 

The Reform leader had been due to appear on the airwaves this morning but was replaced by Richard Tice after being struck down with a heavy cold. 

Mr Tice told the BBC that Mr Trump was worried about China getting a foothold in the Arctic. 'He's correct in that, but the approach in the way you work with your closest allies... is completely wrong,' he said.

He added: 'On this the president has got it wrong.'

Former foreign secretary Jeremy Hunt said he did not believe Mr Trump would follow through on invading Greenland.

'To invade the sovereign territory of a Nato ally would mean the end of Nato... it would dissolve that alliance overnight,' he told the BBC.

However, Mr Hunt acknowledged that it would be 'squeaky bum time' to see whether Europe could hold out. 

Tory peer Lord Hannan said: 'This is demented. Outright batsh** crazy. Where are the adults in the room? Why will no one rein him in?' 

John Bolton, Mr Trump's former national security adviser, said: 'Donald Trump's extraordinary tariff threat against the UK and other countries for things they have said or done regarding Greenland is without doubt his most dangerous and destructive assertion during the five years of his presidency.

'To say it is without precedent radically understates how foolish and contrary to fundamental American interests it is. Its ramifications for the special relationship and Nato alliance – indeed for the credibility and trust the United States has spent decades trying to establish – is incalculable.'

The UK already pays a 10 per cent tariff on some goods imported by the US, after Mr Trump introduced a wave of taxes on countries around the world on his 'Liberation Day' last April.

Mr Trump's announcement is the latest twist in the White House's new foreign policy strategy – dubbed the 'Donroe Doctrine' after the earlier Monroe Doctrine – in which it seeks to dominate the western hemisphere and prevent foreign control of vital assets.

The UK has deployed just one military officer, at Denmark's request, to Greenland to join a reconnaissance group ahead of an exercise named Arctic Endurance.

Mr Trump has insisted the US needs to acquire Greenland for national security reasons and has not ruled out using military force. He has claimed it is vital for his planned missile defence shield known as the Golden Dome.

However, critics claim his interest is linked to Greenland's untapped mineral wealth rather than defence. The island possesses at least 25 of the 34 raw materials considered 'critical' by the EU.

A meeting at the White House last Wednesday, led by Vice President JD Vance with representatives from Denmark and Greenland, ended in an impasse.

Yesterday, thousands of protesters took to the streets in Denmark and in Greenland's capital Nuuk, chanting 'Greenland is not for sale' amid anger over the threats. 

Mr Trump is known to be a huge admirer of the Royal Family. Sir Keir wooed him with an unprecedented second state visit to the UK last year, when he was feted with great pomp and ceremony.

The King has been expected to visit the US in April, while a trip by the Prince of Wales has also been pencilled in.