A Porter Airlines flight diverted from its original route Saturday due to a problem on board – the fourth diversion for the Toronto-based airline in less than a week.

A flight from Billy Bishop Toronto City Airport (BBTCA) that departed for Montreal Saturday morning had to turn around and land again in Toronto after an alarm went off in a washroom.

Porter spokesperson Brad Cicero confirmed the alarm and said the airline is still looking for a “specific cause.”

He said passengers arrived safely in Montreal after switching to another plane.

According to BBTCA’s website, a number of Porter fights scheduled for Saturday have been delayed and cancelled. 

The airport said it was expecting delays and cancellations due to bad weather.

Three Porter flights diverted last Sunday

Saturday’s incident follows three early landings that all occurred last Sunday.

One Porter plane headed from BBTCA to Washington, D.C., was forced to land at a rural Pennsylvania airport after smoke was detected in the cockpit. It arrived safely and passengers were picked up and flown to their final destination.

Just hours later, a Porter flight heading from Thunder Bay, Ont., to BBTCA was forced to land at Toronto's Pearson International Airport.

That evening, a Porter plane heading from BBTCA to Sudbury was forced to make an emergency landing at Pearson after one of the aircraft's two engines shut down.

Porter Airlines uses Canadian-made Bombardier Q400 turboprop planes.