Talks between British Airways and union leaders were brought to an abrupt end when protesters stormed the meeting.
Dozens of demonstrators from the Socialist Workers Party breached security at the London offices of the conciliation service Acas.
It is not known how much progress was made in the last-ditch talks aimed at averting strikes by cabin crew.
The latest strikes follow a long-running dispute over jobs, pay and working conditions.
The two sides sat around the negotiating table for more five hours on Saturday before the protesters interrupted the meeting.
More than 100 demonstrators gathered outside the building, saying they were there to show solidarity with BA cabin crew.
Banners and shouting
Those who made it to the 23rd floor, where the talks were taking place, came face to face with BA chief executive Willie Walsh and Tony Woodley, joint leader of Unite.
Mr Woodley angrily remonstrated with the protesters telling them to "shut up".
The talks had to be abandoned amid scenes of chaos and Mr Walsh had to be escorted by police from the building.
The protesters, many holding up Socialist Workers Party banners, stayed in the building until they were ejected by police officers.
Acas Chief Executive John Taylor said no-one was injured and the parties left the building safely.
"It was disappointing that talks broke up in this way and we are already in touch with the parties to endeavour to restart the discussions," he said.
"On a wider issue, we will also obviously be reviewing security arrangements."
BA cabin crew who are Unite members are set to begin a five-day strike on Monday, with more strikes to follow from 30 May and 5 June.
Before the talks began, Mr Woodley accused BA bosses of having a "petty, vindictive" attitude.
He said the current deal offered by Mr Walsh could not be accepted by his members.
But Mr Walsh, who has said the company will do everything it can to reach an agreement with Unite before Monday, has blamed Bassa, the Unite branch representing crew, for the failure to reach a deal.
"The issue is that the trade union branch at the heart of this dispute - Bassa - has not accepted and will not accept that negotiation is the way forward and that is regrettable but is the truth," he said on Friday.
Mr Woodley denied Bassa was the problem, and said he had recommended the rejection of the latest deal to union members.
"Mr Walsh says that we cannot deliver a deal. He is right about that because the deal on the table cannot be delivered so he has got to change," he added.
'Issue of trust'
BBC business correspondent Joe Lynam said the two sides were very close to reaching an agreement but there was still a big question over the issue of trust.
Chief Acas conciliator Peter Harwood expressed optimism about an agreement, but warned that if a deal was not secured this weekend there was "every possibility that additional pressures on both sides will ensue which will make a final resolution more problematic".
Unite say the main stumbling blocks now are the travel perks that were taken away from members who went on strike in March and disciplinary action which has been taken against more than 50 of its members.
Bassa secretary Duncan Holley said he believed Mr Walsh was "determined to break the union".
Mr Holley, a former BA staffer who worked in cabin crews for 34 years until he was sacked this month after disciplinary hearings, told BBC Radio 4's Today programme that Mr Walsh had never agreed to negotiate with his 12,000 members.
"He keeps talking about us being dysfunctional - we actually are the biggest branch in the country; we represent a third of his workforce," he said.
He added that he did not want to disrupt customers' plans but he had to look after union members.
If the strike goes ahead, BA plans to fly more than 60,000 customers a day next week, operating 60% of long-haul flights and 50% of short-haul services from Heathrow.
It says its services from Gatwick will be unaffected.
The airline is intending to lease up to eight aircraft with pilots and crew from other UK or European carriers.
Earlier this week, BA was granted a court injunction preventing the strikes after the High Court ruled that the Unite union had not reported results of its strike ballot correctly to members.
However, this was overturned on Thursday following an appeal by the union.
On Friday, BA reported its biggest annual loss due to lower passenger numbers, higher costs and the impact of strike action.
The airline lost £531m ($766m) in the 12 months to March - BA's biggest loss since it was privatised in 1987.
It is estimated that strikes by cabin crew in March cost BA £43m.