There has been a setback in attempts to end the dispute between British Airways and its cabin crew, which was triggered by BA cutting staff on some flights.
Leaders of Unite union's Bassa branch - which represents most crew - have said they will not endorse a new peace deal, describing it as "a step too far".
The deal was reached in talks between the joint head of Unite, Tony Woodley, and the BA chief Willie Walsh.
BA says the deal on offer is "fair and reasonable".
It has been called the best that can be achieved by negotiation.
But a Bassa spokesman said: "While there are some good parts within BA's offer... there are many other clauses which we simply as a trade union cannot recommend.
"We simply cannot be held to ransom."
Jerry Hicks, who is running in the election for the general secretary of Unite, said: "It's a dramatic development, it leaves the whole thing [the peace deal] in tatters."
Among the stumbling blocks, Bassa says, are a demand the union gives up all outstanding legal claims arising from the dispute, and an acknowledgement that BA has the right to withdraw travel concessions from staff in future.
Bassa says BA might even withdraw the deal totally as it had been insisting on a favourable recommendation from the union leadership.
The airline's cabin crew workers have staged 22 days of strike action since March, costing the airline £150m.
When the dispute began in November last year, it centred on changes to staffing levels, pay and working conditions.
However, Unite has since said that the core issues are the removal of the travel concessions and the implementation of disciplinary sanctions against its members since March.
BA reported earlier this month that its passenger numbers in September were 1.3% higher than a year earlier.
However, its total traffic for the year to date is still down on a year ago, following the strike action and April's volcanic ash cloud which grounded flights across Europe.