With almost 98 per cent of votes counted, Calderon was more than 400,000 votes ahead of his rival, Andres Manuel Lopez Obrador, preliminary results showed on Monday.

A senior election official said it was unlikely that Obrador could overturn the deficit, despite another count ordered for later this week.

However Obrador, who had claimed victory himself on Sunday, said he would use all legal means to challenge the result.

"We have a commitment to the citizens to defend the will of millions of Mexicans," he said at his campaign headquarters in Mexico City.

Mexico's Federal Electoral Institute has refused to declare a winner until an official count that was to begin on Wednesday.

The delay in announcing a clear winner and the prospect of legal challenges has led to fears of weeks of political deadlock.

'Clear Victory'

"There is an irreversible result and it is in my favour," Calderon, of the National Action Party, said in a television interview.

"The result gives me a very clear victory that cannot be reversed."

A Calderon victory means that Mexico will stick to the free-market policies of the outgoing president, Vicente Fox, and remain a staunch US ally.

During his campaign, Obrador had accused Calderon of pandering to the rich and promised that he would govern for Mexico's 50 million poor.

The stock market jumped 4.8 per cent in afternoon trade and Mexico's peso currency rose about 1.8 per cent, its biggest daily gain in two years.