Transportation Secretary Pete Buttigieg is warning that passengers may be in for flight delays as airlines are poised to miss their 5G deadline.

Verizon and AT&T have been throttling 5G performance around airports after a hard-fought deal in which the carriers made the concession to give airlines time to refit their aircraft. After purchasing billions in mid-band spectrum licenses from the FCC, airlines and the FAA warned that many aircraft altimeters were vulnerable to interference from mid-band 5G.

The two camps agreed to a deal whereby Verizon and AT&T would reduce the strength of their 5G signals around airports, with the understanding they would resume full-strength signal deployment on July 1, 2023.

With that date fast-approaching, it appears that a sizable number of aircraft will not be ready.

“There’s a real risk of delays or cancellations,” Buttigieg said in an interview with The Wall Street Journal. “This represents one of the biggest—probably the biggest—foreseeable problem affecting performance this summer.”

Buttigieg says 80% of the domestic airline fleet and 65% of the international fleet has been retrofitted, but that leave a lot of planes vulnerable to 5G interference.

The 5G rollout has been problematic in the US, with Congress slamming the FCC and FAA’s handling of the fiasco. Unfortunately for travelers, it appears the troubles are far from over.