Its investigation will look into whether BA and three other signatories - American Airlines, Iberia and Finnair - have engaged in anti-competitive behaviour across six transatlantic routes.

The Atlantic Joint Business Agreement was first agreed in 2008. However, it was renegotiated a year later following concerns raised by the EU’s own competition watchdog.

According to the CMA, the investigation is being conducted “under the rules on agreements restrictive of competition”.

The six routes concerned are London-Dallas, London-Boston, London-Miami, London-Chicago, London-New York and Madrid-Miami.

In a statement, the CMA said following an investigation under EU competition law, the four airlines committed to making take-off and landing slots available to competitors at Heathrow and Gatwick for 10 years.

“As 5 of the 6 routes subject to commitments are from the UK, and to prepare for the time when the European Commission may no longer have responsibility for competition in the UK, the CMA has decided to review afresh the competitive impact of the agreement in anticipation of the expiry of the commitments [in 2020],” said the CMA.

It added: “This case is at an early stage and no assumption should be made that the Atlantic Joint Business Agreement infringes competition law.”

In a statement, BA parent IAG said it noted the CMA announcement and would respond to its review.

"Since 2010, British Airways and Iberia’s transatlantic joint business with American Airlines and Finnair has been bringing significant benefits to millions of travellers.

"It provides them with improved access to cheaper fares and easier journeys to more destinations. During this period the joint business has launched 45 new routes including 14 between the UK and US.

"Also, the airlines are able to align their flight schedules and frequencies to enhance customers’ travel choices."