British Airways cabin crew based at the UK's busiest airport have launched a 16 day strike in a long-running dispute over pay.
Members of Unite in the so-called mixed fleet - those who have joined the airline since 2010 - walked out from midnight at Heathrow Airport amid union claims that they are on "poverty pay".
The union accused BA of threatening sanctions against those taking industrial action.
Unite is already pursuing legal action on behalf of around 1,400 workers it says were sanctioned for going on strike earlier this year.
Unite said its action had forced BA to cancel flights and lease aircraft from Qatar Airways.
National officer Oliver Richardson said: "Vindictive threats from British Airways amount to corporate bullying from an airline more interested in punishing workers on poverty pay than addressing why cabin crew have been striking.
"Unite believes it is tantamount to a blacklisting operation and that it is unlawful. We will fight both industrially and legally to defend our members' fundamental human right to stand up to bullying and for decent pay.
"That British Airways is seeking to lease aircraft from an airline found to have breached international standards on labour and human rights, is doubly shameful.
Yet due to the full leasing of nine Qatar aircraft some passengers are getting an upgrading service, instead of travelling on a cramped basic BA A320 seating 177, they get to fly on a Qatar Airways A320 (yep the same plane type) but with one-quarter fewer seats.
Economy passengers were able to enjoy extra legroom and seatback TVs with on-demand movies and music. BA Club Europe passengers are able to lounge around in “deep reclining seats” for their flights.
