Toronto's subway extension into Richmond Hill has taken another step forward with Mayor David Miller's executive committee giving its approval, reports say.

But it's far from a done deal. Miller says he is worried that the cost of the project will be double the forecasted $2.5 billion price tag due to upgrades needed elsewhere along the subway line. He says he wants to make sure all costs are covered by the provincial and federal governments.

The proposed seven kilometre expansion of the Yonge line would stretch into Richmond Hill in York Region, towards Highway 7.

The mayor says the cost of the project should also include renovations to the Yonge-Bloor station, because it will likely need to cope with an increased passenger load.

The executive committee passed a recommendation calling for the city to be freed from any capital or new operating costs for the line that would extend Toronto's subway service to the suburbs.

""If this were built without these requirements being part of it, it would not serve the needs of Toronto," Miller told the Globe and Mail.

Councillor Michael Walker (Ward 22), often critical of the mayor, says the city's demands could derail the project, according to reports.

The plan, strongly supported by politicians in York Region, now needs the final go-ahead from Toronto City Council.