Al Franken has been politically active since leaving Saturday Night Live in 1995. (Canadian Press)

A Minnesota board on Monday certified results showing Democrat Al Franken winning the state's U.S. Senate recount over Republican Norm Coleman, whose lawyer promised a legal challenge that will keep the race in limbo for weeks.

The Canvassing Board's declaration started a seven-day clock for Coleman, the incumbent, to file a lawsuit protesting the result. His attorney, Tony Trimble, said the challenge will be filed within 24 hours.

The challenge will keep Franken from getting the election certificate he needs to take the seat in Washington. Trimble said the process "is now just at the beginning."

Franken, a former Saturday Night Live personality and writer, ended the recount up by 225 votes, an astonishingly thin margin in a race where more than 2.9 million votes were cast. The recount reversed the unofficial Election Day results, which showed Coleman with a 215-vote lead.

Franken made up the deficit over seven tortuous weeks of ballot-sifting in part by prevailing on challenges that both campaigns brought to thousands of ballots. He also did better than Coleman when election officials opened and counted more than 900 absentee ballots that had erroneously been disqualified on Election Day.

Coleman says recount unfair

Coleman's lawyers have argued that some ballots were mishandled and others were wrongly excluded from the recount, giving Franken an unfair advantage. After a Minnesota Supreme Court decision went against Coleman earlier Monday, lead attorney Fritz Knaak said a lawsuit was inevitable.

A lawsuit would extend the fight over the seat for months.

Secretary of State Mark Ritchie was careful to note that the board was simply signing off on the numbers found by the recount: Franken, with 1,212,431 votes, and Coleman, with 1,212,206 votes.

"We're not doing anything today that declares winners or losers or anything to that effect," Ritchie said.

All five members of the canvassing board — Ritchie, plus two state Supreme Court justices and two Ramsey County judges — voted to accept the recount results.

Schumer declares race settled

Lawyers for both campaigns have laid the groundwork for lawsuits through public comments and legal manoeuvring. Any court case would open doors closed to the campaigns during the administrative recount. They would be able to access voter rolls, inspect machines and get testimony from election workers.

New York Senator Charles Schumer, who until recently was the head of the Democratic senatorial campaign committee, said Sunday the race was settled and that Franken had won the election.

"While there are still possible legal issues that will run their course, there is no longer any doubt who will be the next senator from Minnesota," Schumer said. "With the Senate set to begin meeting on Tuesday to address the important issues facing the nation, it is crucial that Minnesota's seat not remain empty, and I hope this process will resolve itself as soon as possible."

Senator John Cornyn, the chairman of the national Republican senatorial committee, called Schumer's comments premature and troubling, since Schumer is the new chairman of the Senate rules committee, which has jurisdiction over contested elections.

"Senator Schumer will likely play a key role in determining who ultimately assumes this Senate seat," the Texas senator said. "Prejudging the outcome while litigation is still pending calls into question his ability to impartially preside over this matter when it comes before the committee, as it most certainly will."

Coleman's term as senator expired Saturday.

Senate Republican leaders have said the chamber shouldn't seat Franken until all legal matters are settled, which could take months.