Britain's competition regulator is looking into a partnership between three airline groups which covers transatlantic routes.
The UK Competition and Markets Authority (CMA) is investigating International Airlines Group (IAG), American Airlines and Finnair.
IAG is the parent company of Aer Lingus, as well as British Airways and Spain's Iberia.
The CMA said its investigation has been delayed as a result of the COVID-19 pandemic.
It launched a competition investigation into the Atlantic Joint Business Agreement in October 2018.
The body said this was "in line with the approach of the European Commission when it first investigated the agreement during 2009 to 2010".
In 2010, the European Commission accepted commitments from the parties in relation to six routes between Europe and the USA to address potential competition concerns.
These included a commitment to make landing and take-off slots available to competitors at either London's Heathrow Airport or London's Gatwick Airport.
These commitments were binding for 10 years.
The European Commission could have re-assessed the agreement when the commitments expired in 2020, but there was no requirement for it to do so.
The CMA has said as five of the six routes are from the UK, it decided to "review afresh the competitive impact of the agreement in anticipation of the expiry of the commitments."
The CMA added that there is "no assumption" that the Atlantic Joint Business Agreement infringes competition law.
Over the next six months the watchdog is going to examine the tie-up in relation to the recovery of the aviation market post-COVID.