BC Ferries is warning travellers to be ready for long line-ups over the B.C. Day long weekend. (BC Ferries)

Travellers are being warned to expect long line-ups at all major BC Ferries terminals over the B.C. Day long weekend, the busiest weekend of the year for the ferry system.

By Friday night evening, most sailings from mainland B.C. to Vancouver Island had an average of a one-sailing wait.

By 5:30 p.m. PT Friday, all vehicle space on the Tsawwassen-Southern Gulf Islands route had been fully booked.

BC Ferries spokesperson Deborah Marshall estimated as many as 250,000 passengers would board a ferry over the long weekend.

She advised commuters to check the current conditions on the BC Ferries website before heading out to the terminals.

She also cautioned travellers to bring plenty of water in order to stay hydrated while waiting in long line-ups.

BC Ferries also released its list of the busiest sailings over the B.C. Day long weekend.

Officials said most sailings from the mainland to Vancouver Island were expected to be packed on Friday and Saturday, while Monday and Tuesday were expected to be busy in the opposite direction as travellers head back from the Island.

Border crossing busy

Traffic heading into the United States was also busy on Friday evening, with long line-ups at two major B.C. border crossings.

A one-hour delay was reported at both the Pacific Highway and Peace Arch border crossing.

Officials are also warning commuters to expected long line-ups heading back into B.C. on Monday.

On the roads, Highway 37 was closed in both directions from the Beaverdam rest area to just south of the Yukon border because of a forest fire. Motorists were advised to take Highway 97 instead.