But he added that the Zimbabwe Electoral Commission (ZEC) might also decide to delay the vote.
"It is again provided for in the constitution that it has to be done within 12 months. But as far as I can see from this, they're going to announce the run-off anytime from now."
On Friday, the MDC said it had been cheated of thousands of votes as the commission announced that Mr Tsvangirai had gained 47.9% of the votes - against 43.2% for Mr Mugabe.
The party now faces a dilemma, says the BBC's Peter Biles in Johannesburg.
It can boycott the run-off and hand victory to President Mugabe, or it can take part - under protest - in the knowledge that the violence and intimidation witnessed over the past month has created a far tougher electoral environment, our correspondent says.
Timing
Zimbabwean Deputy Information Minister Bright Matonga told the BBC a second round could be held within three weeks.
But he added that the Zimbabwe Electoral Commission (ZEC) might also decide to delay the vote.
"It is again provided for in the constitution that it has to be done within 12 months. But as far as I can see from this, they're going to announce the run-off anytime from now."
On Friday, the MDC said it had been cheated of thousands of votes as the commission announced that Mr Tsvangirai had gained 47.9% of the votes - against 43.2% for Mr Mugabe.
The party now faces a dilemma, says the BBC's Peter Biles in Johannesburg.
It can boycott the run-off and hand victory to President Mugabe, or it can take part - under protest - in the knowledge that the violence and intimidation witnessed over the past month has created a far tougher electoral environment, our correspondent says.
But he added that the Zimbabwe Electoral Commission (ZEC) might also decide to delay the vote.
"It is again provided for in the constitution that it has to be done within 12 months. But as far as I can see from this, they're going to announce the run-off anytime from now."
On Friday, the MDC said it had been cheated of thousands of votes as the commission announced that Mr Tsvangirai had gained 47.9% of the votes - against 43.2% for Mr Mugabe.
The party now faces a dilemma, says the BBC's Peter Biles in Johannesburg.
It can boycott the run-off and hand victory to President Mugabe, or it can take part - under protest - in the knowledge that the violence and intimidation witnessed over the past month has created a far tougher electoral environment, our correspondent says.
