British Columbia's political leaders exchanged duelling letters over the future of the Site C dam project on Tuesday, with Premier Christy Clark arguing that delays will cost hundreds of millions of dollars.

NDP Leader John Horgan wrote to BC Hydro last week asking it to suspend the evictions of two homeowners and urging it not to sign any new contracts on the $8.8-billion hydroelectric dam until a new government has gained the confidence of the legislature.

But Clark says in letters sent to Horgan and Green Leader Andrew Weaver that the evictions are necessary as part of a road and bridge construction project that are needed to divert a river in September 2019.

Any delay could postpone the diversion by a year and cost taxpayers $600 million, she says.

"With a project of this size and scale, keeping to a tight schedule is critical to delivering a completed project on time and on budget," she says. "The requests contained in your letter are not without consequences to the construction schedule and ultimately have financial ramifications to ratepayers."

The premier has asked Horgan and Weaver to reply by Saturday on whether they still want to put the evictions on hold. She says a decision to proceed must be made by June 15 in order to maintain the river diversion schedule.

She also asks whether they want the government to issue a "tools down" request to BC Hydro on other decisions that she says are essential to maintaining the budget and construction schedule.

In his reply, Horgan writes he was surprised to receive the letter from Clark.

"In it, you made unsupported claims about additional costs associated with asking BC Hydro not to sign major contracts until a new government takes office," he says.

"If you are truly concerned about this timeline, there is a simple solution: recall the legislature immediately and face a confidence vote so British Columbians can get the new government they voted for."

An agreement between the NDP and Green party was signed last week that would allow the New Democrats to form a minority government, ousting Clark's Liberals.

The agreement includes a promise to refer the Site C project to the B.C. Utilities Commission to determine its economic viability.

But Clark says the project is likely to progress past the "point of no return" before a review can be completed.

Clark didn't define what she meant by "point of no return," nor did she explain how she reached the $600-million figure in her letter. Her press secretary Stephen Smart referred questions to BC Hydro, which declined to answer them and instead sent an emailed statement.

"BC Hydro is aware of the letters sent by Premier Clark to the leader of the B.C. NDP and the leader of the provincial Green party. We have no further comment at this time," it said.

Weaver issued his own letter that says before he can comment on Clark's assertions he requires access to supporting evidence, including signed contracts, the project schedule and potential alternative project timelines.

"Your government is turning a significant capital project that potentially poses massive economic risks to British Columbians into a political debate rather than one informed by evidence and supported by independent analysis."

The dam will be the third on the Peace River, flooding an 83-kilometre stretch of valley, and local First Nations, landowners and farmers have fiercely opposed the project.

Construction began two years ago and the project employs more than 2,000 people.

A report by University of British Columbia researchers in April argued it wasn't too late to press pause on the project and that the electricity produced by Site C won't be fully required for nearly a decade after it's complete. It said cancelling the project as of June 30 would save between $500 million and $1.65 billion.

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