British Airways cabin crew are said to be strongly supporting the start of a 16-day strike over pay.
Members of Unite employed in the so-called mixed fleet walked out today, mounting picket lines at Heathrow Airport.
BA said it planned to operate 99.5% of its schedule.
The firm said: "Our oneworld partner Qatar Airways will be operating a small number of short-haul flights on our behalf.
"We have merged a very small number of Heathrow long-haul services and all customers affected have been notified over the past week."
Unite national officer Oliver Richardson said: "We are seeing strong support from members of British Airways' mixed fleet cabin crew who are determined to take a stand against poverty pay and the airline's vindictive sanctions.
"The time and money British Airways has spent on wet leasing aircraft and bullying striking cabin crew are resources which could have easily settled this dispute.
"Instead, British Airways has spent millions defending low pay. The world is watching a once great brand become associated with inequality and bullying.
"Unite is currently assessing its legal options regarding British Airways' wet leasing of aircraft from Qatar Airways and would urge the airline to see sense and drop its bullying approach."
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