British Airways is planning to take legal action to stop the latest strike by its cabin crew, the company said.
The action comes despite recent pressure for talks from both the conciliation service Acas and the pilots' union, Balpa.
BA will seek an injunction in the High Court on Monday, just 24 hours before the first of four five-day walkouts is due to be held.
Transport Secretary Philip Hammond said he was surprised by the move.
BBC business correspondent Joe Lynam, who spoke to Mr Hammond, said the transport secretary was perplexed by the letter, given that he is set to meet both sides in this dispute on Monday morning anyway.
He said he had not been informed of BA's intention to go to the courts.
Earlier on Friday, Acas made a public offer to both sides to hold talks, a move that appeared to meet with approval from the two.
Both BA and the Unite union responded by saying they were prepared to hold talks.
BA explained its move to the courts, saying: "The Trade Union and Labour Relations (Consolidation) Act 1992 requires unions to send everyone eligible to vote details of the exact breakdown of the ballot result."
It went on: "We do not believe Unite properly complied with this requirement. We wrote to the general secretaries of the union yesterday [Thursday], asking them to explain to us how they discharged this obligation and, based on Unite's replies, we believe that they failed to comply with the legal requirement."
A Unite spokesman said: "We will be vigorously defending our ballot and members against this move by BA. We have already responded to the company, and notified them that we have fully complied with the law."
The spokesman added that the answer lay with talks: "The only way to settle this long-running dispute is through negotiation. A solution is not to be found in the courtroom."
Acas said its public offer came because of its concerns about the length of the current dispute between the company and the Unite union and its effect on the travelling public.
The British Airline Pilots Association (Balpa) has also appealed for talks.
It has asked the government to bring pressure to bear.
Balpa said it had written to the transport secretary and Business Secretary Vince Cable, warning that a wave of strikes from next week would risk jobs.
Mr Hammond has urged them to "hammer out a solution".
Cabin crew will walk out for four separate five-day strikes in May and June.
The first strike is scheduled to begin on 18 May, ending on 22 May, with the three further strikes beginning on 24 May, 30 May and 5 June.
Unite members rejected a fresh deal for cabin crew earlier this month.
Balpa said ministers should work to bring BA boss Willie Walsh and Unite leader Tony Woodley together in a bid to get next week's strikes suspended.
"The last three days have changed the face of British politics with erstwhile political opponents coming together for the common good," said Balpa general secretary Jim McAuslan.
"We call on government to use that political momentum to help solve what are tired 1970s-style industrial relations.
"Without some of that Cameron/Clegg magic, this dispute will put the future of a great airline at risk and disrupt the lives of thousands of British families."
Mr Hammond told the BBC that BA risked being weakened as an airline.
"My call to both the unions and the management today is for people to get back to work," he said.
"Let's cancel this strike, let's sit down around the table and hammer out a solution in the interests of passengers, in the interests of the UK economy... and if I may say so, in the long-term interests of the workers involved in this dispute."
The Unite union said there was a turnout of 71% of the cabin crew it represents, and 81% voted against BA's offer last week, which sought to resolve the dispute over pay and working conditions.
Details of that deal were not given, but Unite said BA had failed to restore the travel perks it withdrew from staff involved in the previous strikes in March.
Those stoppages grounded hundreds of flights and cost BA an estimated £45m.
The newly-announced strikes will see staff walk out for a total of 20 days, covering spring bank holiday and school half-term holidays.
England's first World Cup game against the United States kicks off in South Africa on 12 June, after the final strike has ended.
BA has defended its approach to the latest negotiations, saying it had made a fair offer and that the decision to strike showed "a callous disregard" for BA customers and staff.
All flights from Gatwick and London City airports are expected to operate as normal, BA said, while a "substantial" number of long-haul flights from Heathrow will run as planned.
There will be daily flights from Heathrow to all short-haul destinations, BA said, but did not give any numbers.
As with the strikes in March, the airline is also planning to lease extra aircraft from other carriers and re-book passengers on cancelled flights onto other airlines.