Tony Blair has told Sky News he thinks it would be “deluded” not to realise that Brexit would “diminish Britain’s position on the world stage”.
Mr Blair also said that more needed to be done to maintain a healthy relationship with the United States, despite his own disagreements with Donald Trump, because Brexit could make the UK “dependent” upon America.
The former prime minister is attending the World Economic Forum in Davos, where he has been a regular visitor in recent years.
He says he now believes there is “a crisis in the liberal order, which is why you are getting an explosion of populism from the right and left”.
Mr Blair sees Brexit as part of that response and remains an avowed opponent of any move to leave the European Union.
He said he considers leaving the EU “a big economic risk” that would lead to years of uncertainty – and that it will also damage the stature of Britain.
“If we do leave, it will take a long time to restructure the economy,” he said.
“There is no doubt that leaving Europe will diminish Britain’s position on the world stage.
“Britain will remain an important country. If it leaves the European Union it will still count – it will be a major economy, with political weight – but it will be diminished.
“You only have to factor in how the relationship with the United States would change.
“If Britain pulls out of Europe then even our relationship with America will not be as powerful, because the US has often looked to Britain as its big ally with the European Union.
“You are pulling out of a big regional bloc on your doorstep, just at a time when many big countries are coming together.
“You can’t be deluded about it – it will diminish our standing.”
Image: Mr Blair says the US-UK relationship must remain strong
Mr Blair also has clear views on how the UK should manage its relationship with Donald Trump: “You have to handle this in the following way – when he does or says something we disagree with then you have to be honest about that, but you have to keep the relationship strong.
“The relationship is stronger than any one administration.
“What is important, especially if you go through with Brexit, is that the American relationship becomes ever more important. In fact it even becomes dependent.
“So if there are disagreements, for example on climate change, it shouldn’t interrupt that relationship.
“Britain and America have big things that we share and the important thing is the areas, like security and the Middle East, where Britain and the US will work together.
“If you focus the relationship around the things we have in common then that is the best way to handle it.”
Mr Blair is not the only Labour politician in Davos; shadow chancellor John McDonnell is attending the forum.
When I asked Mr Blair about that, he responded with a wry smile, describing Mr McDonnell’s decision to attend as “an indication that the Labour Party has changed in a fairly fundamental way”.
But he remains unconvinced by the party’s approach to Brexit.
“I understand what Jeremy Corbyn is saying, which is that the country has voted to leave Europe and that means leaving the single market.
“But the other side, which John McDonnell talks about, is that we have to remain part of the European single market.
“I think those two things are in conflict so the Labour Party will have to decide if it is going to prioritise the necessity, in my view, of staying part of European market.
Image: Shadow chancellor John McDonnell is also in Davos
“If it does, and Labour starts to advocate the position of retaining a large part of regulatory alignment with Europe then people will say ‘if we are still going to abide with Europe’s rules why are we leaving the European Union and losing our seat at the decision-making table?’
“That’s why it is a binary choice – if you are going to leave because you want to separate then you have to leave single market and customs union and that will be a big economic risk for the country and it will then take a long time to restructure the economy.”
Mr Blair knows that his old party is the favourite to win the next election. So does he share that view? Does he think Jeremy Corbyn will end up in Downing Street?
“I don’t know – I think it’s really hard to predict.
“I think the Brexit factor is a huge distortion of British politics at the moment. I think the Conservatives believe that if they deliver Brexit they will stay in power.
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“I actually think that Brexit is their biggest problem because there is a whole section of the population that might not be inclined to vote Labour otherwise, but will vote Labour because of Brexit.
“And the Labour Party has got a big set of decisions to make as this negotiation unfolds as to whether they back the Government or stake out their own decision.”