Donald Trump risked alienating the United States from its allies after launching a scathing attack on world leaders at the G7 summit.

The US president strained relationship with the rest of the G7 was clear on Saturday when he turned up late for the opening session, missing Canadian Prime Minister Justin Trudeau’s opening remarks.

Trump also flouted the event dress code, snubbing Trudeau’s request for leaders to wear smart casual attire to create a more informal setting, opting instead for his usual suit and tie, reports the Mirror.

He then left the summit early to head for his talks with North Korean leader Kim Jong-un, but not before blasting the world’s biggest economies in a bad-tempered press conference.

Despite rows over trade and Russia, Trump brazenly rated his relationship with other G7 leaders as 10 out of 10.

He told reporters: ‘I would say that the level of relationships is a 10 - we have a great relationship. Angela (Merkel) and Emmanuel (Macron) and Justin (Trudeau) - I would say the relationship is a 10.’

He added: ‘The relationship that I’ve had with the leaders of these countries - I would really rate it, on a scale of zero to 10, I would rate it a 10.

‘That doesn’t mean I agree with what they are doing, and they know very well that I don’t.’

To round off an extraordinary day, Trump later branded Trudeau ‘dishonest and weak’ in a Twitter tirade in which he withdrew his endorsement of the joint communique at the end of the G7.

He tweeted: ‘Based on Justin’s false statements at his news conference, and the fact that Canada is charging massive Tariffs to our U.S. farmers, workers and companies, I have instructed our U.S. Reps not to endorse the Communique as we look at Tariffs on automobiles flooding the U.S. Market!’

Trump continued: ‘PM Justin Trudeau of Canada acted so meek and mild during our @G7 meetings only to give a news conference after I left saying that, ‘US Tariffs were kind of insulting’ and he ‘will not be pushed around.’ Very dishonest & weak. Our Tariffs are in response to his of 270% on dairy!’

The summit in Canada was marked by Trump’s controversial ‘America First’ trade policy which has put him at odds with the rest of the G7 over his steel tariffs move against the EU.

European leaders called Trump’s decision ‘illegal’ and a ‘dangerous game’ and is now sizing up a revenge attack with duty on US bourbon, jeans and Harley-Davidson motorbikes unless he backs down.

And at showdown trade talks at the G7 summit on Friday, May blasted the US president’s actions which have hit its ‘strongest allies’.

But yesterday, Trump hit back as he warned that retaliating to American steel tariffs would be a ‘mistake’.

Ramping up the global trade war, the US president accused other states of ‘robbing’ his country through their trade policies.

The US has slapped a 25% tariff on imports of steel and 10% on aluminium from countries including the UK and the rest of the EU.

Trump yesterday told reporters the US needed protectionist tariffs because ‘we are like the piggy bank that everyone is robbing’.

May pleaded with fellow leaders to step back from the brink of a damaging trade war at a session of the summit on Friday night.

The UK is thought to have concerns about the European Commission’s proposed package of £2.46 billion of tariffs on US goods.

Trump said: ‘If they retaliate they are making a mistake.’

As well as criticising the tariffs, May also opposed Trump’s call for Russia to be readmitted to the group of leading industrialised nations.

But Trump insisted it would be an ‘asset’ to have Vladimir Putin back at the summit table.

The Prime Minister said: ‘Where we disagree with our allies on something it is right that we say so and air the issue openly and frankly.

‘We have done just that at this summit, registering our deep disappointment at the unjustified decision by the US to apply tariffs to EU steel and aluminium imports.’

She continued: ‘In response, the EU will impose counter-measures.’

But ‘at the same time we need to avoid tit-for-tat escalation’ and maintain a constructive dialogue, she said.

The Prime Minister said the UK still wanted to discuss a post-Brexit trade deal with the US and said Mr Trump was still looking forward to July’s visit to Britain.

‘We have a very good relationship with President Trump, we work with President Trump, the UK has a very good relationship with the US,’ she said.

May added: ‘President Trump and I had a brief word here at the summit about how he is looking forward to his visit to the UK and I look forward to welcoming him to the UK.’

The US president repeated his call for Vladimir Putin to join the other leaders around the summit table.

‘I think it would be an asset to have Russia back in, I think it would be good for the world, I think it would be good for Russia, I think it would be good for the United States, I think it would be good for all the countries of the current G7,’ he said.

‘I think the G8 would be better, I think having Russia back in would be a positive thing.’

Russia was thrown out of the group in 2014 over the annexation of Crimea.

The Prime Minister has been at the forefront of diplomatic efforts to build an alliance against Russia after the Salisbury nerve agent attack on ex-spy Sergei Skripal and his daughter Yulia.

She said: ‘If there are to be any conversations about Russia’s future in relation to this particular group, they have to change their ways, they have to change the route that they are taking and the sort of activity they are involved in.’