The ex-Prime Minister said it could be either through a vote in Parliament, an election or a second referendum.
He made his comments after writing an article which issued a rallying cry for Remain voters to mobilise, organise and “prise apart the alliance which gave us Brexit”.
Mr Blair argued that during the referendum people were yet to see the alternative future they were committing to, but said it would become clearer as Brexit negotiations began.
He said: “If you want to retain that access to the single market there will be various obligations that are imposed upon you, in relation to the free movement of people, to legal obligations…you are going to have to work out at that point, ‘are the freedoms that we’re going to enjoy…really so substantial that we want to leave the European Union?’.
”Another possibility is that you actually go for a much harder form of Brexit, you leave the single market altogether…then you’re going to be able to calculate, how much pain, how much difficulty, economic/social restructuring, is going to be necessary to make a success of that.“
Speaking to BBC radio, he went on: ”If it becomes clear that this is either a deal that doesn’t make it worth our while leaving or alternatively, a deal that is going to be so serious in its implications, people may decide that they don’t want to go, there’s got to be some way, either through Parliament or through an election, possibly through another referendum, in which people express their view. “
Writing in the New European newspaper, Mr Blair highlighted the sharp fall in the pound as a ”negative prediction about our economic future“, and the blocking of a Canada-EU free trade deal by the Walloon regional parliament in Belgium as evidence that Brexit may not take the form many Leave voters had imagined.
He said that Brexiteers are envisaging ”free market, free trading, light regulation, low tax, low social protection“ like in city states Hong Kong and Singapore, something he claimed not all Leave voters would back.
He said Remain voters must win the argument that staying in the EU should stay on the table, warning that anger and anxiety about immigration, globalisation, stagnant incomes, housing and squeezed public services will only get worse outside the EU.
”Right now there is one point and one point only to win: we should keep every option open,“ Mr Blair wrote.
”That this should even be contentious speaks loudly about how much those of us, and after all we were 16 million people, who believe Britain's future lies within the European partnership, have been shoved on to the defensive.“
He said people supporting Remain are ”the insurgents now. We have to build the capability to mobilise and to organise. We have to prise apart the alliance which gave us Brexit.“
Join our commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies