A former top aide to Karen Stintz, running in her old ward, has broken with the mayoral candidate over transit for Scarborough, calling a subway extension there fiscally irresponsible.

Jean-Pierre Boutros was at the side of Ms. Stintz during the long and protracted battles at council over transit in Scarborough. He was willing to accept a subway in the area until the cost became unacceptable, he said.

"There is better service to be had with that money [for] Scarborough and Toronto, this is not the way to do it," the ward 16 candidate said in an interview. "You have [a light-rail plan] that is approved, that has been vetted by professionals, by transit experts, by city planners and by sensible people. Why choose a plan B?"

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Mr. Boutros said he is speaking out now because of his hope that Premier Kathleen Wynne, secure in her new-found majority, will have the confidence to accept that light rail makes more sense for the area. As for the effect of his position on his former boss, he said that it can have an impact only if Ms. Stintz and the other pro-subway candidates are unable to defend their position.

"I believe it is an election issue," Mr. Boutros said. He dubbed the property-tax hike that will pay for part of the transit expansion the Scarborough Subway Tax (SST) and used it to draw a parallel with the unpopular Vehicle Registration Tax (VRT) that helped get Rob Ford elected.

"We're taking almost $700-million from the federal government, we're taking a billion [dollars] from us, we're taking $1.5-billion from the provincial government … all of this money doesn't need to be spent."

Mr. Ford and mayoral challengers John Tory and Ms. Stintz want to go ahead with the subway plan. Front-runner Olivia Chow, as well as David Soknacki, prefer light rail in Scarborough.

Rapid transit in Scarborough has been a dominant issue at Toronto council and Queen's Park for more than a year. After much debate, the promise of a $1.8-billion light-rail line in Scarborough, paid for entirely by the province, was spurned by city council in favour of a subway extension that will cost much more. The province would contribute less to the new project, with the remaining price-tag covered with $660-million promised by Ottawa and $1-billion raised from Toronto residents.

To subway-boosters, the debate is over. But critics say the project remains in flux. And much work remains.

As The Globe reported Saturday, the ridership projections are very preliminary and won't be refined until next year. Also, there has been no formal application for the federal contribution and the environmental assessment has not begun. The existing contract for a light-rail line – a deal signed in 2012 between the city, the TTC and the regional transit agency Metrolinx – is still being renegotiated. There is no completion date in sight for those talks and the original master agreement remains in force.

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"We have something on the books that's signed right now, that's good to go," Mr. Boutros said. "Let's do it."

As recently as late last month, Ms. Wynne was asked a few times about Scarborough transit and chose not to state definitively that the province's contribution could be used only for a subway.

"It is the plan that is in place but, as with all of these things, we have to work in co-operation with the municipalities," the Premier told reporters. "There is, you know, there's no point in pretending that there doesn't have to be a good working relationship."