Viviane Reding made the assertion Tuesday during an appearance with Chantal Gaemperle, a board member at French luxury label LVMH who signed an EU-sponsored pledge to increase the number of women in the company, EUobserver.com reported.
The commissioner said she would look to see whether companies have made a "clear, precise and measurable evolution" about achieving gender balance by March.
"If yes, there will be no need for European legislation. If no, we will have a problem with the internal market," she said, explaining that major companies within the European Union would be confronted with different national laws on women quotas.
France, Spain, Belgium, the Netherlands and Italy already have rules about gender balance in business.
Reding, who said she wants to boost female boardroom positions to 30 percent by 2015 and to 40 percent by 2020, said she is "completely supported" by internal market commissioner Michel Barnier.
Women currently make up 10 percent of directors and 3 percent of chief executive officers in the EU's largest companies, EUobserver.com said. The number of women sitting on boards increasing by about a half-percentage point each year.