The former prime minister said the next 12 months will be crucial as 2017 was “too early in the negotiation” and by 2019 it will be “too late” to have a say on whether Britain’s future relationship with Brussels is better than the current arrangements.
In the latest in a string of interventions since the June 2016 referendum, Mr Blair attacked Labour leader Jeremy Corbyn, saying the party’s “timidity” over Europe under his leadership will usher in Brexit if it continues.
Mr Blair’s comments quickly provoked the ire of Brexit supporters.
The campaign group Leave Means Leave said voters would take no notice of a leader “who took us to war on a lie and who cannot admit when he is wrong”, the former accusation a reference to controversial Iraq War in 2003.
In his call for his successor to radically alter his approach, Mr Blair urged Labour to “make Brexit the Tory Brexit”.
The ex-PM, who led Britain from 1997-2007, said he disagreed with Mr Corbyn’s position “strategically”, adding he believed it was “mistaken” tactically.
“First, because the Labour Party is saying that we too would do Brexit, we cannot attack its vast distractive impact,” Mr Blair said.
“Labour could mount such a powerful assault on the Government’s record from the appalling state of the NHS to crime, which through neglect and failure to support the police is on the rise again, if we were saying to the country: here’s the agenda which could be delivered for the people were it not for the fact that all the energies of Government and substantial amounts of cash are devoted to Brexit.
“And, second, it puts us in a vulnerable position when the Government concludes ‘the deal’ some time in 2018.”
Mr Blair said Labour should instead “fight for the right for the country to re-think” and demand full details from the EU of what the future relationship will look like before finalising Brexit.
He suggested going “to the high ground on opposing Brexit and go after the Tories for their failures to tackle the country’s real challenges”.
Mr Blair added: “At every PMQs nail each myth of the Brexit campaign, say why the Tory divisions are weakening our country, something only credible if we are opposed to Brexit, not advocating a different Brexit, and challenge the whole farce head on of a Prime Minister leading our nation in a direction which even today she can’t bring herself to say she would vote for.
“If we do leave Europe, the governing mind will have been that of the Tory right. “But, if Labour continues to go along with Brexit and insists on leaving the single market, the handmaiden of Brexit will have been the timidity of Labour.”
As well as his article, the former PM’s Institute for Global Change has issued a report setting out what it says has been learned since the referendum.
Liberal Democrat Sir Nick Clegg, a former deputy PM and supporter of the pro-EU campaign group Open Britain, said the document laid bare the “eye-watering costs and sheer complexity” of Brexit.