The French government has warned 500 companies they face fines for failing to comply with gender equality laws.

Women's rights minister Najat Vallaud-Belkacem said the firms had been given an ultimatum to comply with anti-discrimination legislation dating back several years, but were still dragging their heels.

She said five companies were already being fined "penalties of several million euros for each month … until they bring themselves in line with the law".

"It is no longer a virtual sword of Damocles hanging over the heads of enterprises, but a real one. Suddenly the companies are sending us their professional equality plans. More than 4,000 have done so since January," Vallaud-Belkacem told Europe 1 television.

"But to be efficient, it [the legislation] has to be dissuasive and above all plausible," the minister, who has said that fining businesses who break the law is a "last resort".

French companies employing more than 50 people have been required to move towards equality in both seniority and salary since 2010, but many paid lip-service to the principle. However, in January 2012, financial penalties were introduced for non-compliance – legislation that allows the government to fine companies up to 1% of their total wage bill.

The gap between men's and women's salaries in private companies in France is estimated at 27%.

One of the companies that has already been fined, which employs 150 people, had a €500 a month average difference between male and female workers' salaries. It was fined €5,000 a month, 1% of its total wage bill, until it introduces more pay equality.

Asked about the company earlier this year, Vallaud-Belkacem, who has refused to name it, said: "The firm presented us with a plan that proposed nothing to remedy the situation."

Another firm, with 150 staff based in Aquitaine, south-west France, that failed to react to a government ultimatum was ordered to pay €8,500. It was sold shortly afterwards. "It's now up to the new owners to comply with the law," she added.

A European commission report in 2012 found that French women had to work 79 days more than men to gain the same salary.