LONDON — A planned European Union law to impose sanctions on companies that do not allocate at least 40 percent of the seats on their boards to women has drawn enough opposition from Britain and other countries to risk being blocked before it is officially proposed.

Nine E.U. countries have signed a letter to the European Commission, the Union’s executive agency, opposing the proposed law, which is due to be published in draft form next month.

The proposal has been championed by Viviane Reding, vice president of the commission, who has been pressuring European companies since last year to improve the representation of women in top management or risk having penalties imposed on them.

The signatories argue that although barriers to success for women in E.U. companies are “unacceptable,” national governments should determine what sanctions, if any, should be applied to companies that fail to improve.