Theresa May could have to delay the March 2019 Brexit date because of the political impasse over the UK's future outside the EU, Tony Blair will say.
The former prime minister will use a major speech to hit out at both the Tories and Jeremy Corbyn's Labour over the state of the Brexit process, demanding a more assertive Parliament and a fresh referendum on any deal with the EU.
His comments come in a speech in which he will warn about a rise of populism which risks a return to the 1930s and the danger of the West losing sight of the values which brought the US and Europe together.
He will tell the Chatham House foreign affairs think tank that he has "never been more worried about the future of our country" as a result of the Brexit process.
"We should plan now for the possibility we need to extend the March 2019 deadline," he will say.
"Presently, we are drifting towards March 2019 with no clear negotiating position, no resolution of the Northern Ireland question, still vaguely hoping Europe will allow us access to the single market without abiding by its rules which it will never do, and with senior Cabinet members openly debating the merits of a negotiating position which 'threatens' Europe with a no-deal Brexit.
"This is the equivalent of holding a negotiation on the top floor of a high-rise building and 'threatening' to jump out of the window if our demands are not met.
"The whole thing has become so protracted that it has numbed our outrage."
The Prime Minister is "more a hostage than a leader" as a result of the competing factions in the Conservative Party.
Mr Corbyn, meanwhile, "neglects to lead the fight here at home over an issue which literally determines the future of Britain".
Mr Blair will say: "Parliament must assert itself because neither Government nor Opposition can or will.
"Then the people must make the final decision because only they have the right to decide what version of Brexit they want or whether in the light of all they now know they prefer to remain."
Mr Blair will use his speech to stress the importance of the transatlantic alliance, despite the difficulties caused by Donald Trump's administration.
The former Labour leader will say: "There are disputes, whether over trade, commitment to Nato spending, how to tackle the Middle East or climate change. Friends can disagree.
"But we need to know from the current American administration and its president that our alliance matters."
If that assurance is given, European leaders "need to respond in kind".
Mr Blair will acknowledge that supporters of globalisation are on the back foot as left and right-wing populists argue against free trade, migration and international alliances which are "portrayed as contrary to putting national interest first".
"Once it is clear the populism isn't working because, ultimately, it offers only expressions of anger and not effective answers, the populists may double down, alleging that failure is the result of half-heartedness and that only more of the same will work," he will warn.
"Who knows where the dynamic of that scenario takes us. Then the comparisons with the 1930s no longer seem far-fetched."
Calling for "new thinking and muscularity in defence of reason", Mr Blair will warn: "We are losing sight of the values which brought the West together, saw it through the menace of fascism and communism and, for all our justifiable grievances, have wrought immense progress."