Malcolm Watson agreed to a three-year sentence at his home in Canada. ((CBC))

An American teacher that a U.S. judge sentenced to three years of probation in Canada after he pleaded guilty to sexually assaulting a student was arrested by Canadian border guards on Thursday.

Malcolm Watson was arrested while returning from a court appearance in Buffalo, N.Y., officials with Public Safety Minister Stockwell Day's office confirmed.

"We don't want to see Canada become a haven for, whether it's pedophiles or any other person committing a serious crime, and we don't want U.S. courts getting the notion that we just take people here that they would have put in jail but instead they sent him to Canada," Day told reporters in Ottawa.

Watson, who was convicted of sexually assaulting a 15-year-old student at a New York state school,lives in St. Catharines, Ont.

He was sentenced to serve three years probation in Canada after he requested to live in Ontario with his Canadian wife and children. A U.S. judge approved the plea bargain.

The unique deal has set off a political firestorm at both the federal and provincial levels.

Ontario Premier Dalton McGuinty has slammed the American court for approving the plea bargain, saying Canada is not a "dumping ground" for U.S. offenders.

Watson was ordered by the U.S. court to provide a DNA sample and is forbidden from re-entering the U.S., except for probation hearings.

Day said Thursday there was "clear language" in existing Canadian legislation that would render Watson inadmissible in the country.

Permanent residents can be deported for serious crimesthat would garner sentences of 10 years or more in Canada, including crimes that are considered misdemeanours in the U.S.

Watson is scheduled to appear at an immigration hearing on Friday which will determine if he remains detained, with the government next week arguing why he should beprevented from staying.

If Ottawa bars him from entering Canada, his sentence will be declared null and void, and he will be forced to return to the U.S., though Watson will have the right to appeal.

Niagara Regional Police had been reviewing his conviction, and reasons and terms for his probation order, as well aswhether he had any history with police in St. Catharines or Fort Erie, where he previously resided.

With files from the Canadian Press