Germany's top parties, in talks on forming a government, agreed on Monday to a binding boardroom quota for women intended to fix a big gender imbalance in business.

Chancellor Angela Merkel's conservative Christian Democrats (CDU) and their likely coalition partners, the Social Democrats (SPD), said they planned to require listed companies to fill at least 30 percent of the seats on their supervisory boards with women from 2016.

The accord will be set down in a manifesto for a left-right government, which the parties are expected to hammer out this month, with the aim of eventually passing new legislation.

As the fraught negotiations between the traditional political rivals enter their final phase, the two sides also agreed on a "flexi-quota" for executive boards and upper management.

This would make companies listed on a stock exchange set their own "binding goals" for more top jobs for women.

The SPD's chief negotiator in the social issues working group, Manuela Schwesig, said the decision sent "an important message about improving women's opportunities to get ahead."

Annette Widman-Mauz of the CDU called it a "reasonable solution in women's interest".

A lack of women at the helm of companies has long been a contentious issue in Europe's top economy but the conservatives had resisted setting binding quotas.

Women held only four percent of the seats on the executive boards of Germany's top firms last year, in a country where 68 percent of women work and more than a third of members of parliament are female.

Merkel, Germany's first female chancellor, had repeatedly called on corporations to improve their gender ratios on a voluntary basis but warned that if they failed to make progress, the state would take action.

The same coalition negotiating committee agreed overnight to improve paid parental leave options after the birth of a child, and to offer 10 days paid leave to care for an ailing family member.

However the Social Democrats failed to win support for a proposal allowing gay and lesbian couples to adopt children, and to divert subsidies for stay-at-home parents to public child care facilities.