An accidental "customs breach" at Pearson airport Wednesday afternoon caused a long delay for passengers at U.S. customs in Terminal 3.

A WestJet plane flying in from Calgary offloaded its passengers accidentally onto the U.S. side by an open bypass door. Guests then needed to clear U.S. customs and return to the Canadian side.

For security reasons, U.S. custom services were suspended in Terminal 3, resulting in long lineups for boarding passengers. 

"A customs breach occurred at Terminal 3, US Departures at 15:36 when passengers arriving from an inbound domestic flight were inadvertently given access to a bypass door to the US departures area," said Beverly MacDonald, a Toronto Pearson spokesperson. 

"In order to ensure the security of departing flights, boarding was held while the situation was investigated and some passengers to be reprocessed through U.S. Customs," she said.

Police received a call just after 4:30 p.m. on Wednesday, said Const. Harinder Sohi.

WestJet spokesperson Lauren Stewart said the plane was an inbound flight from Calgary.

The breach was resolved just after 5 p.m. and passengers were able to board their planes.