An erroneous downgrade of French sovereign credit rating by Standard & Poor's rating agency on Thursday was a very serious incident and will have to be investigated, the European Commission said.

'This is a very serious incident. This shows that we are in an extremely volatile situation, that markets are extremely tense, and therefore players on these markets must be extremely rigorous and exercise a duty of responsibility,' remarked EU internal market commissioner Michel Barnier on Friday.

The Standard & Poor's mistake spooked investors already anxious over Europe's worsening debt crisis, feeding concerns that Europe's debt problems had engulfed the region's second-largest economy.

It contributed to the worst day for France's government bonds since before the euro was launched in 1999.

In a statement, S&P said the message resulted from a technical error and not from any action it intended to take against France. It said it was investigating the cause of the mistake.

'It is all the more important since these are not minor players on these markets, but actually one of the three major rating agencies and therefore an agency that has a particular responsibility,' Barnier said.