Approximately 80,000 vehicles travel every day over the Pattullo Bridge, which spans the Fraser River between New Westminster and Surrey, B.C. (CBC)

The Pattullo Bridge, which crosses the Fraser River to connect New Westminster and Surrey, B.C., could be closed for as long as a month after a fire damaged the bridge's support structure Sunday.

A spokesman for TransLink, the organization responsible for public transportation, roads and bridges in the Lower Mainland, said the fire damaged a 20-metre section of wooden trestles at the south end of the bridge early Sunday morning.

"It's heavily charred," Ken Hardie said. "When the blaze first broke out … reports were that there were flames shooting up from underneath the bridge on both sides."

The cause of the fire was not immediately known, but investigators are treating it as suspicious.

The bridge, which carries an average of 80,000 vehicles across the Fraser River every day, will be closed to vehicle and foot traffic in both directions Monday, Hardie said.

He warned public transit riders to expect delays as there will be additional demand on SkyTrains and bus routes for those travelling between Surrey and New Westminster, Burnaby and Vancouver.

Hardie said motorists throughout the region should expect additional traffic congestion and longer travel times, especially on the other three bridges that span the Fraser River: the Queensborough Bridge, the Alex Fraser Bridge and the Port Mann Bridge.

To avoid congestion, TransLink is advising commuters to avoid peak travel times, between 7 and 9 a.m. PT.

TransLink has plans to eventually tear down the 71-year-old bridge and replace it with a six-lane, $800-million structure, but B.C. Transportation Minister Kevin Falcon said Sunday a new bridge is still many years away.