The European Union prolonged pandemic-induced regulatory relief for airlines, only partially reimposing a requirement that they use their airport slots when a 12-month waiver ends next month.

The bloc will make airlines take up at least 25% of their takeoff and landing positions during the summer season or risk losing them the following year, down from a normal 80% level. The new regime will run from March 28 through October, according to legislation approved Thursday.

The new law replaces the EU’s full suspension of the “use-it-or-lose-it” slot rule to help airlines as border closures and lockdown restrictions wiped out travel. Air traffic in Europe has tumbled over the past 11 months and was down more than 60% in January compared with a year earlier.

The waiver was put in place to avoid a scenario in which empty planes fly routes just so operators can retain their slots, which are worth millions of dollars. The EU last suspended the rule in 2009 as a result of the financial crisis.

The effective threshold for airlines this summer will vary carrier to carrier, depending on whether they opt to return slots and if so how many. The minimum 25% level would result from carriers being allowed to return half of their slots and being required to use at least half of the remaining ones.

Under the new law, the European Commission, the bloc’s regulatory arm in Brussels, will have the power to fix the threshold at between 30% and 70% for the subsequent two seasons lasting into next year. Many airlines have forecast a gradual pick-up in demand from later in 2021 as vaccine rollouts start to take hold.