Four people were taken to hospital with minor injuries after a massive pileup in the westbound lanes of Highway 401 in Milton, Ont. on Wednesday.

Sgt. Kerry Schmidt, spokesperson for the OPP's highway safety division, said the collision involving more than 20 vehicles occurred on Highway 401 west of Highway 25 in Milton. Halton paramedics were called to the scene at about 12:40 p.m.

"We've got at least 15 passenger vehicles, maybe seven more transport trucks, and we've got about 10 kilometres of backed up traffic," Schmidt said shortly after the collision occurred.

"This is going to be a major delay, a major cleanup."

All of the injured were taken to a local hospital, according to a Halton Region official.

The pileup occurred on Highway 401 westbound in the Milton, Ont. area, west of Highway 25 near the Kelso Conservation Area. (Sgt. Kerry Schmidt/Ontario Provincial Police/Twitter)

All westbound lanes of Highway 401 in the area were initially closed, but all lanes have since been reopened after several heavy and light duty tow trucks straightened out and moved vehicles.

Transport trucks had jack-knifed at the start of the pileup. The collision occurred west of the Kelso Conservation Area.

Several emergency crews, including paramedics and firefighters, were at the scene. Paramedics treated people outside their vehicles.

"It just keeps going on and on and on. Unfortunately everyone's going to have to wait and be patient," Schmidt said.

'It's certainly a bad day for a lot of folks."

All lanes are now open <a href="https://twitter.com/hashtag/Hwy401?src=hash&ref_src=twsrc%5Etfw">#Hwy401</a> WB west of <a href="https://twitter.com/hashtag/Hwy25?src=hash&ref_src=twsrc%5Etfw">#Hwy25</a> <a href="https://twitter.com/hashtag/Milton?src=hash&ref_src=twsrc%5Etfw">#Milton</a> —@OPP_HSD

The OPP, with the help of Halton Regional Police, are investigating the cause of the pileup. Weather may have been a factor because the roads are slippery, Schmidt said.

Motorists were urged to avoid the area and the pileup caused major traffic delays.

"We are assisting with traffic once it gets off the highway," Const. Ryan Anderson, spokesperson for Halton police, told CBC Toronto before the roads reopened.

"The drivers are flooding the roads of Milton," he added.

Christine Barber, Halton Region deputy chief of logistics, paramedic services, said the injuries sustained by the four people were non-life threatening.

Highway 401 was closed westbound from James Snow Parkway to Guelph Line.