Tony Blair has urged European Union leaders to work to stop Brexit - warning them that the UK could act as a focus for further disunity in the bloc if it leaves.
The former British prime minister said Brexit would damage the economies of both the UK and EU and weaken the bloc's "standing and power" on the world stage.
He called for reform - particularly to deal with concerns on immigration - as a way of persuading British voters to reverse Brexit.
In a speech in Brussels, Mr Blair said the economic cost of Brexit to the rest of the EU would be "significant and painful".
And he warned: "Britain out of Europe will ultimately be a focal point of disunity, when the requirement for unity is so manifest.
"No matter how we try, it will create a competitive pole to that of Europe, economically and politically to the detriment of both of us.
"More contentiously, I believe it risks an imbalance in the delicate compromise that is the European polity."
Brexit would be "a divorce that can never mean a physical separation".
"We are consigned to cohabiting the same space, trying to get along but resenting our difference and re-living what broke us apart, awkward silences at the breakfast table, arguing over the rules with no escape from each other.
"But - and here is the supreme irony - with so much in common and still liking each other."
It was, he said, "better to make our future work together".
Ahead of the speech, Mr Blair had said Prime Minister Theresa May's Brexit plan is "literally not going to happen" no matter how tough she is in negotiations with the EU.
Mr Blair said Britain is no further forward nearly a year after triggering Article 50 to start negotiations.
He said there is a fundamental dilemma at the heart of Brexit which there is "no way round" - the UK can either leave the single market and customs union to take control of its laws but take an economic hit and a hard border in Northern Ireland, or stay in and keep a frictionless frontier and easy trade with the EU.
Mr Blair told BBC Radio 4's Today programme: "The problem that she (Mrs May) has is that there is no way round the dilemma.
"What she thinks is that it's possible to get the European Union to give us access to Europe's markets without the same obligations that the rest of Europe has in the single market.
"That is not possible. It's not a question of a tough negotiation or a weak negotiation, it literally is not going to happen.
"So the dilemma you have is you're either going to have to stay close to Europe to minimise economic damage, in which case you abide by Europe's rules, or you're free from Europe's rules, in which case you're going to have economic damage."
Mr Blair repeated his criticism of Brexiteers who dismiss the impasse in negotiations over maintaining a soft Irish border.
"I find it not just disappointing but sickening that people should really be prepared to sacrifice peace in Northern Ireland on the altar of Brexit," he said.
He said Jeremy Corbyn's commitment to a customs union with the EU to maintain a soft border and maintain tariff-free trade was "sensible", but warned that Labour will "very soon find that we've got to move further in order to escape the dilemma ourselves".