Iran's main reformist party announced on Thursday that it would go ahead with a boycott of key polls in two weeks, charging that a review ordered by supreme leader Ayatollah Ali Khamenei had resulted in the reinstatement of just 51 of some 2,500 candidates blacklisted.
The Guardians Council - the conservative-dominated vetting body which ordered the original blacklist - insisted its review was still under way and more candidates might yet be reinstated.
But reformist MPs, who have already seen much of their legislative programme vetoed by the Guardians, made clear their patience was at an end.
Some 130 deputies, who have been holding a sit-in at parliament since January 11, announced they would now make good their threat to resign their seats and were consequently abandoning their original protest.
"It's the worst possible outcome," said Islamic Iran Participation Front leader Mohammad Reza Khatami, brother of pro-reform President Mohammad Khatami, as he announced his party's decision to boycott the February 20 parliamentary elections.
"According to the information available to us, they (the Guardians) have reinstated 51 candidates, eight or nine of them sitting MPs", he said.
As a result, "the Front's decision to boycott the polls remains unchanged."
The Guardians Council touched off a political storm last month when it barred some 3,500 out of 8,000 candidates from standing in the elections, most of them reformers and including 87 sitting MPs.
An earlier intervention by Khamenei secured the reinstatement of some 1,000 candidates, but a raft of key reform figures, including Mohammad Reza Khatami, remain barred.
The mass disqualification of so many of their candidates has brought reformers' frustrations to a head after years of deadlock with the Guardians, an unelected constitutional watchdog body which has the right to block new legislation as well as candidates for public office.
The council's deputy chairman, Reza Zavarehi, insisted "the review is not over.
"The Guardians Council has until February 9 in the evening" to announce its decision," he told the student news agency ISNA.
But reformist MPs were in no mood to give the council the benefit of the doubt.
"In the light of the news that this illegal process is continuing without fundamental change, we cannot take part in the February 20 elections which will not be free but will be unfair and unjust," said a statement issued by the organisers of the sit-in.
"We unmasked a parliamentary coup which had been in preparation for two years but the body which organised this coup is still hoping to carry it off by semi-legal means," the MPs said.
One of the protesting deputies, Jalil Sazegarnejad, said MPs would now abandon the sit-in and "continue our struggle by other means."
The lack of movement from the Guardians came despite earlier positive comments from Khamenei about the prospects for a settlement.
"There is no knot that cannot be untied ... as shown by the revolution which has overcome every obstacle placed in its path," Iran's supreme leader said Wednesday.
Both sides had made clear that Thursday would be a crucial day in the burgeoning political crisis, which many now regard as the most serious in the Islamic regime's 25-year history.
"After the meeting with the guide, we hope for acceptable results by tomorrow that will allow the organisation of the elections," a spokesman for the reformist-led government said Wednesday.
"The limit, for us, is tomorrow afternoon," said the spokesman, Abdollah Ramezanzadeh.
Earlier Thursday, the moderate president met with representatives of the 2nd of Khordad Front, a coalition of 18 reformist groups which back him.
But the deadlock between his supporters and the Guardians has set the scene for a showdown with the supreme leader.
Despite his calls for a compromise solution, Khamenei ordered Wednesday that the government must go ahead with the organisation of the elections.
He also warned that resignations by officials in protest at the barring of candidates were "against the law and prohibited by Islam" and could incur heavy penalties.