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The proportion of those surveyed supporting the change in the law rose to 63 percent from 60 percent in early January and December.
Support for adoption rights for gay couples also rose by 3 percentage points, although the country remains divided on the issue, with 49 percent in favour, according to the international marketing firm.
The French government underlined its determination to press ahead with a reform bill earlier this month even after roughly half a million people marched through Paris on Jan. 13 to show their opposition to the proposal.
"We have observed that the proportion of people in favour of marriage and adoption has risen three points, compared with our previous survey, done before the large demonstration," said Ifop pollster Jerome Fourquet. "Support for the plan is increasing - particularly on the Left."
Thousands marched in the French city of Lyon on Saturday in support of "marriage for all" ahead of a similar demonstration planned in Paris on Sunday.
The latest Ifop poll was based on the views of 1,026 people aged 18 or over and was carried out between Jan. 22 and 24, Ifop said.