Meanwhile, a three-day strike by Air France cabin crew over working conditions has been planned for 28 March to coincide with Unite's action.
A spokesman for six French unions told the Daily Telegraph they would do anything they could to support BA's staff.
In Germany, an official from the ver.di service workers union said: "We want Unite to succeed, if it emerges from these talks that something is needed at this or that airport at this or that time, we will do it."
BA claims up to 1,000 employees have offered to stand in for those who strike and expects that number to rise further.
The airline said it was "pleased" the number of other airlines offering to help fly passengers during the strike had increased from 50 on Monday to more than 60.
Chief executive Willie Walsh said: "I am delighted by the numbers of cabin crew who have been getting in touch with us to express their disillusion with Unite's position.
"Our crews just want to work as normal, do their usual terrific job and look after our customers.
"We will now have the potential to fly more than 4,000 additional customers per day and serve more destinations."
But Unite accused the management of "bluffing" and said only 350 volunteers were prepared to stand in for the strikers.
It also wants the government to investigate the "rushing through" of volunteer staff.
BA says the majority of the extra staff were existing cabin crew employees, not retrained office staff.
There have been no indications of any fresh talks planned between the two sides in the dispute.
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