Germany's foreign minister says Berlin will fight a European Union proposal to introduce mandatory quotas for the number of women on the boards of private companies.

The plan proposed by European Commission Vice-President Viviane Reding in November would require at least 40 percent of the non-executive directors of publicly traded European companies with more than 250 employees to be women by 2020.

But Foreign Minister Guido Westerwelle says it's a national issue that Brussels shouldn't be involved in - a stance also taken by Britain.

He says: "Germany will not only not accept such a directive, but we will work actively against it."

The proposed measure still needs approval by EU leaders and the European Parliament.