Using its powers under the Euratom Treaty, the Commission is expected to send a letter to the French government demanding access to all safety information relating to geological damage and radioactivity. According to a text being discussed in Brussels, the letter will instruct France to hand over the information within five days "and in any case before any further tests in the current series are carried out".
Although two tests have been carried out, the Commission has until now declined to invoke European law, which gives it the power to assess health and safety measures before any "particularly dangerous experiment" is carried out by a member state.
Jacques Santer, the Commission President, has been reluctant to provoke a clash with France, in view of the potential political backlash for the EU. The Commission's nuclear experts advised Mr Santer to send a verification team to inspect safety measures in the South Pacific as long ago as July, but none was sent until after the first test.
French lobbying against intervention by the Commission has been intense. Last week it was revealed in a secret diplomatic memorandum that France was confident it had an understanding with Mr Santer that the Commission would not take action.
However, several factors have brought about a tougher Commission stance. There has been mounting public criticism, reflected in an increasingly belligerent attitude from the European Parliament, which debates the issue tomorrow. The parliament has warned that it might take the Commission to the European Court of Justice for failing in its duty as a "guardian of the treaty".
The Commission initiative has also been provoked by anger at France's failure to hand over sufficient information on safety measures in the South Pacific.
That has intensified in recent days, since Commission officials who visited the region were refused access to the test sites. Nuclear experts working in Brussels have raised serious concerns about potential danger from the tests, warning that cracks in the rock strata could allow leaks of radiation.
The Commission hopes that the letter to Paris will bring about a political compromise. However, if enough safety information is not provided or it does not prove satisfactory, the issue could be taken to the European Court of Justice.
The EU's Euratom Treaty provides the only legal avenue for a challenge to the nuclear testing. Previous tests carried out during the Cold War provoked less concern from the anti-nuclear lobby, and the treaty has rarely been invoked. With the end of the Cold War, however, Greenpeace, supported by many member states, has raised the issue.
The powers of the Commission to challenge France come under Chapter Three of the treaty, which gives Brussels responsibility for protecting the health and safety of workers and the general public against radiation. Article 34 states: "Any member state in whose territories particularly dangerous experiments are to take place shall take additional health and safety measures, on which it shall first obtain the opinion of the Commission."
The Commission has already requested information from the French on health and safety monitoring, but key data has been withheld.