BRUSSELS — Companies allocating fewer than 40 percent of seats on supervisory boards to women could face serious sanctions later this decade, according to a proposal made Monday by Viviane Reding, the European Union justice commissioner.

Ms. Reding has long campaigned for major changes in European boardrooms, and last year she gave industry a final opportunity to improve its record on placing women in top management.

In March, she said self-regulation had failed and that legislation would be required in order to accelerate gender equality in many of the most senior areas of business life.

If approved by her colleagues at the European Commission in the coming weeks, Ms. Reding’s proposal would require state-owned companies to name women to 40 percent of the seats on supervisory boards by 2018, according to a summary of the draft proposal seen by the International Herald Tribune.