Sir John Major has also backed the idea of a second referendum before Britain leaves, in order to give the public a say on the deal struck by the Government and EU countries.
Both former prime ministers campaigned for the UK to remain in the EU, and fear Theresa May's Government will withdraw Britain from much of the single market in a so-called "hard Brexit".
In a magazine interview, Mr Blair said: "It can be stopped if the British people decide that, having seen what it means, the pain-gain cost-benefit analysis doesn't stack up...
"When I say, 'Well, let's just keep our options open', it's condemned as treason. Why wouldn't you keep your options open?
"Why wouldn't you say, 'We took this decision, we took it before we saw what its consequences are; now we see its consequences, we're not so sure?'"