Shadow chancellor John McDonnell is to join picket lines at Heathrow today as British Airways cabin crew begin a three-day strike over “poverty pay”.

Union members of the airline’s so-called mixed fleet will also be joined by Unite general secretary Len McCluskey.

The escalating dispute at BA came as Unite claimed victory in negotiating an improved pay deal for 4,000 crew at Virgin Atlantic

The pay agreement implemented from October 2016 means cabin crew will a get 6.65% increase and flight service managers and cabin service supervisors will get a 4.45% rise.

This will be followed by a 3% increase or RPI plus 0.5%  – whichever is greater – from December this year.

The deal also includes an average 18% increase across all ranks on flight pay for Virgin crew, according to the Union.

McCluskey said: “I am very pleased that Virgin has worked with Unite to ensure the crew’s contribution to the success of the airline has been recognised.

“The negotiations were long and sometimes hard but they were constructive and fair.

“With the looming threat of inflation and an uncertain economy following Brexit, this pay deal spanning two and a half years will be a big help to the Virgin crew.”

The 72-hour stoppage at BA follows last week’s two-day industrial action, which the union says saw hundreds of mixed fleet cabin crew stay away from work and mount picket lines around Heathrow.

The airline pledged to fly all passengers to their destinations over the three days but is merging a “very small number” of short haul flights to and from Heathrow.

This will result in just one per cent of all scheduled flights being cancelled.

BA plans to run all long haul flights from Heathrow as normal while services to and from Gatwick and London City airport will be unaffected.

Unite estimates that on average mixed fleet cabin crew earn £16,000, including allowances, a year, while basic pay starts at £12,192 with £3-an-hour flying pay.

The airline disputes this figure and says all mixed fleet crew are paid more than £21,000.

Unite national officer Oliver Richardson said: “British Airways needs to drop its confrontational stance which is causing so much anger and leading to plummeting morale among its mixed cabin crew.

“Rather than trying to bully workers and focusing its resources on leasing aircraft to cover striking cabin crew, British Airways should focus its energies on trying to resolve our members’ legitimate concerns over poverty pay.

“Unite urges British Airways to wake up to the deepening anger of mixed fleet cabin crew and start valuing their contribution by meaningfully engaging with Unite to address poverty pay.”