TORONTO - Ontario Premier Dalton McGuinty is putting federal Liberal Leader Michael Ignatieff on the spot about tax harmonization.

It's important for Ontario residents to know where the federal Liberals stand on the blending of the provincial sales tax with the federal GST, McGuinty said Wednesday.

"I had the clearest of impressions that the federal Liberal party was to be supportive of our efforts to move ahead with a single sales tax in the province of Ontario," he said Wednesday.

"There appears to be some doubt now associated with that, so I will be seeking clarification at the earliest possible opportunity."

The premier maintains that he received assurances from his federal cousins that they would support the HST if they formed the next government.

Ignatieff's camp has denied that he had a deal with the premier, but wouldn't clarify his stance on the HST, which Ignatieff has publicly derided as the "Harper Sales Tax."

McGuinty says he will move ahead with the single tax no matter what.

The two Liberal leaders appear to be out of sync on the blended tax, which has sparked controversy because it will raise the cost of many items currently exempt from provincial tax.