As the biggest-ever Christmas getaway begins, passengers travelling through Britain’s largest airports face the prospect of disruption.

Even though 25 December is still nine days away, the rush for the sun has started. Heathrow airport has told The Independent that its busiest day is 16 December; for the remainder of the year, passengers at the UK’s main business airport can expect a calmer experience.

But there is still no indication about whether or when the Unite union will call a strike of British Airways cabin crew who work for “Mixed Fleet”. They are staff who have joined BA since the 2010 strikes, and comprise around 28 per cent of the airline’s total cabin crew. They work exclusively to and from Heathow; no other London base is affected.

Unite has 2,500 members in Mixed Fleet, and describes their earnings as “poverty pay”. The union’s regional officer, Matt Smith, said: “Mixed Fleet crew earn just over the minimum wage. Significant numbers of crew are taking on second jobs, many go to work unfit to fly because they can’t afford to be sick.

“British Airways bosses need to wake up to the anger and the injustice here.”

In a strike ballot in which 60 per cent of members voted, 79 per cent were in favour of industrial action. The union is obliged to give seven days’ notice of industrial action, which makes it highly unlikely that Christmas flights will be affected.

BA said it is “extremely disappointed” that the strike call “is creating uncertainty for our customers”. The airline added: “We have a range of contingency plans in place and will operate the vast majority of our flights as planned if Unite acts upon the result of this ballot.”

The Independent has identified 59 routes to and from Heathrow that are believed to be operated by Mixed Fleet. Of these, 29 are short-haul, including links with Manchester, Gibraltar and Prague. The remainder are long-haul, including Miami, Cape Town and Singapore.

Possible contingency arrangements include deploying staff who trained as cabin crew during the 2010 dispute, and switching passengers booked on Mixed Fleet flights to nearby destinations — such as Los Angeles instead of San Diego, and San Francisco rather than San Jose in California.

Disruption continues on the ground, with links to Gatwick airport, the nation’s second-busiest, once again hampered by a strike by train drivers working for Southern. At Victoria station in London, the only train running on the main line towards Brighton was the Gatwick Express. With a service only every half-hour, rather than the usual 15-minute frequency, passengers were given a "disruption discount" — paying only the £15.50 Southern fare rather than the usual £19.90 for the Gatwick Express.

Although the strike ends at midnight, further disruption is expected on Monday and Tuesday when conductors working for Southern will go on strike. In addition, a continuing overtime ban by drivers is likely to have an impact on services.

National Express, the coach operator, has added extra services between London and Brighton, and reports it is experiencing its busiest-ever day on the route.