The company announced Tuesday that it has the BC Ferries commissioner's approval to upgrade the Swartz Bay-to-Tsawwassen route ships.
BC Ferries says it expects to save about $9.2 million annually by switching the two Spirit Class vessels to LNG because they account for 15 per cent of the fleet's total fuel consumption.
It also has plans to build three dual-fuel vessels for the southern Gulf Islands and the Powell River-to-Comox route, with completion set for 2017.
The company says all five vessels are set to be operational by 2018, with the Spirit of Vancouver Island upgrades expected to be finished before those of the Spirit of British Columbia.
BC Ferries spokeswoman Deborah Marshall says the cost of the projects is not yet known and that contracts are expected to be awarded next year.