* Move follows Ontario decision to aid GM

* Resulting spike in deficit forecast pressures rating (Adds details)

By Frank Pingue

TORONTO, June 2 (Reuters) - Rating agency DBRS said on Tuesday it lowered the trend on Ontario's long-term debt rating to "negative" from "stable" due partly to the province's move to help bailout automaker General Motors Corp GM.N.

The decision by DBRS comes one day after the Canadian province said the cost of bailing out GM will cause it to run a deficit of C$18.5 billion ($17 billion) in the current fiscal year, 31 percent more than it forecast in March.

"The cost of the initiative to the province is substantial and considerably dampens the fiscal outlook revealed in the March 2009 budget, which was already a significant source of concern for Ontario's credit profile," Eric Beauchemin, DBRS managing director, public finance, wrote in a note.

DBRS said that while Ontario's economic diversification and moderately low debt burden are positive factors, the province's ability to weather the recession and auto sector crisis without unduly weakening its credit metrics are a concern.

The rating agency said its decision to lower the trend could eventually lead to a rating downgrade for Canada's most populous province, a move that would likely increase its borrowing costs.

"It's not a certainty that the rating will get downgraded but it's obviously a sign of increased probability that the rating could be downgraded eventually," Beauchemin told Reuters.