The former prime minister said the party’s leadership was committed to Britain’s exit from the European Union “whether for reasons of opportunism or covert opposition” to the bloc.
In a speech in Speaker’s House in Parliament, Mr Blair said the party he once led cannot be relied upon and urged MPs to defy party whips.
Labour’s leader expressed Eurosceptic views before taking the helm of the party and critics claimed his efforts in the referendum campaign were lukewarm.
Mr Blair, a long-standing critic of Mr Corbyn, said only Parliament can give voters the final say once the settlement has been negotiated.
“We cannot rely on the Government,” he said. “It has been plain for a long time that their primary interest, given the divisions, is to keep the facade of unity.
“Unfortunately, we cannot rely on the Opposition because its leadership believes, whether for reasons of opportunism or covert opposition to the EU, that they must commit to doing Brexit but pretend that they would secure a better Brexit deal.”
Mr Blair stepped up calls for a second EU referendum and claimed Britain “will have to take what we are given”.
“By the end of 2020, the transition will end,” he said.
“The cliff edge will beckon. We can navigate a harder or easier descent; but retreat will be impossible.
“It is this strategy that Parliament has a duty to foil. It has demanded a ‘meaningful vote’.
“The vote is only meaningful if it is on a proposition which allows us to know with precision what our future path looks like before we take it.
“Exposing the strategy of fudge and preventing it, should be the overriding aim of the Labour Party in Parliament.
“I understand, though don’t agree, with its decision to go along with Brexit.
“But it is the duty of Opposition MPs to thwart a strategy designed to place the country in a position where it puts beyond reach of reconsideration a decision of this fundamental importance whose full consequences we do not know.
“Failure to stand against the fudge would be unforgivable.”
Mr Blair said the Conservatives believe delivering Brexit “is the best inoculation against a Corbyn Government”.
“Brexit is not the route to escaping a Corbyn Government; it is the gateway to having one,” he added.
Mr Blair’s latest intervention comes after Owen Smith was sacked from Mr Corbyn’s shadow cabinet after calling for a second public vote, risking deepening divisions within Labour over the approach to Brexit.