Delta Air Lines and Aeromexico will launch their joint venture a little later than planned, as they work to finalise the paperwork for the immunised partnership.

The carriers are still finalising some of the details of what they call a “joint cooperation agreement” with regulators in Mexico and the USA, a spokeswoman for Atlanta-based Delta says.

The tie-up was to launch on 1 April, said Delta chief executive Ed Bastian in January.

Now, the joint venture is expected to begin this spring, or before the end of the second quarter, says the spokeswoman.

Delta and Aeromexico received final antitrust approval for the partnership in December 2016. The immunity allows them to coordinate schedules and fares, share revenues and expenses, and jointly market flights in both Mexico and the USA.

The airlines agreed to divest 24 slot pairs at Mexico City airport and four pairs at New York John F Kennedy airport as a condition for approval.

Delta owns a 36.2% stake in Aeromexico, which closed earlier this month. The US carrier has the option to raise this to 49%.

Both carriers are members of the SkyTeam Alliance.