The strike, by 2,900 members of the Unite trade union, is over a pay dispute for members of the mixed fleet.
They are workers recruited since 2011 and the union has said they are underpaid.
BA has said most Heathrow flights will operate as normal, with a small number merged, but all flights to and from Gatwick and City Airport should operate as normal.
A strike planned for Christmas Day and Boxing Day was suspended, before the new 48-hour action was announced after crew rejected a new offer aimed at solving the dispute.
Unite national officer Oliver Richardson said: “More and more mixed fleet cabin crew have joined Unite as this dispute over poverty pay has gone on."
He added: “Despite British Airways walking away from meaningful talks we would urge it to listen to its mixed fleet cabin crew who work tirelessly to serve passengers and contribute massively to the success of the airline."
The airline said: "As part of our contingency plan, we will be merging a very small number of flights to and from Heathrow. This will mean some customers will travel slightly earlier or later in the day than their original booking."
It added that it looked forward to welcoming 200,000 customers onto flights over the next two days.