Theresa May's Brexit plan is "literally not going to happen" no matter how tough she is in negotiations with the European Union, Tony Blair has said.
The former prime minister 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.
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Ahead of a major Brexit strategy speech from the Prime Minister on Friday, Mr Blair defended his predecessor as PM, Sir John Major, who has said MPs should have a free vote on the final deal with the option of putting it to the public in a second referendum, describing his intervention as "heartfelt" and "analytical".
Ahead of his own speech in which he will urge European leaders to help stop Brexit by promising new measures to address concerns on immigration, 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 Labour will "very soon find that we've got to move further in order to escape the dilemma ourselves".
The former premier was visiting Brussels to ask for assistance in preventing the UK's departure from the EU, telling the continent's leaders they share the responsibility to "lead us out of the Brexit cul-de-sac".
Mr Blair will tell the European Policy Centre think tank: "Reform in Europe is key to getting Britain to change its mind."
He will call for "a comprehensive plan on immigration control, which preserves Europe's values but is consistent with the concerns of its people and includes sensitivity to the challenges of the freedom of movement principle".