At least 25 close calls have happened on Pearson International Airport runways in the last five years, including another runway incursion this week, says the Transportation Safety Board.​

The latest incident happened at roughly 6:35 p.m. on Monday and has prompted a TSB investigation, said Ewan Tasker, Ontario's regional manager of air investigations with the board.

Republic Flight 3553, a regional United flight flying in from Newark, N.J., was landing at Pearson, he said. The plane was cleared to land on Runway 24 Left — an outer runway. After landing, the crew was told to not cross the inner runway that's normally used for departures. At the same time, Air Canada Flight 878 was departing from the inner runway, en route to Zurich.

As the plane on runway 24 Left was approaching the inner runway line, Tasker said the air traffic controller noticed they were going faster than normal and told them to stop, but the plane wound up crossing the line into the protected area.

"If that incident happened in isolation, we probably wouldn't be launching a full investigation into it," Tasker said. "But we've had this type of exact incident happen a whole bunch of times recently."

This marks the second similar incident at Pearson in less than a week. The TSB also deployed a team of investigators to the airport on Aug. 9 after another runway incursion.

The latest close call happened amid a broader TSB investigation into runway incursions between the parallel runways at the south complex of Pearson.

At least 25 runway incursions since June 2012

That investigation, launched in May, is examining numerous incidents that have happened between runways 06 Left and 24 Right, and 06 Right and 24 Left, between June 2012 and June 2017.

Tasker said in total, at least 25 runway incursions have happened at Pearson since June 2012.

"Eventually, one of them could be much more serious," Tasker said.

Back in June 2012, the Greater Toronto Airport Authority — which operates Pearson — wrapped up a previous investigation into runway incursions on the south complex of the airport, according to the TSB.

They found 40 occurrences of interest between 2004 and the end of 2011, of which 20 were inner runway incursions.

"The GTAA implemented a runway incursion plan based on its findings, and completed the changes outlined in the plan at the end of 2013," reads a summary from the TSB.

In a statement to CBC News, the GTAA said they are "fully cooperating" with the TSB's ongoing review. 

The changes made by the GTAA in 2013 included lighting, markings and signage improvements, and additional information for flight crews "to support situational awareness of runway incursion 'hot spots' at Toronto Pearson," coupled with an awareness campaign with the U.S. regional carriers.

"While these improvements have had a significant impact, incursions in this area continue and we'll be looking to mount this campaign again for continued awareness," the statement continued.

The GTAA will also be asking NAV Canada — the capital corporation that owns and operates Canada's civil air navigation service — to meet and discuss ways to reduce incursions.

The TSB's ongoing investigation is happening through the fall, and a final report will be made public after that.