The European Commission opened on Monday (19 September) a case over tax rulings granted by Luxembourg to French gas company Engie that it says amount to illegal state aid.

Engie, which was at the time called GDF Suez, is currently owned at 33 percent by the French state.

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The tax rulings the commission is investigating were first issued in 2008 and allowed two GDF Suez companies to lend money to two other GDF companies, with the four entities avoiding taxes due in normal cases.

The two borrowers could make provisions for interest payments, even while the loans were with zero interest, and file them as tax-deductible expenses.

For the lenders, the loans were converted into shares in the borrowing firms. The benefits were not taxed because in Luxembourg equity investments are exempt from taxation.

"A single company cannot have the best of two worlds for one and the same transaction," EU competition commissioner Margrethe Vestager said in a statement.

"Financial transactions can be taxed differently depending on the type of transaction, equity or debt," she pointed out.

The tax rulings allowing the schemes "contradict national taxation rules and allow GDF Suez to pay less tax than other companies", she added.

The decision to open the case comes as Vestager is starting a visit in the US, where the administration and businesses have strongly criticised the commission's sate-aid stance.

The US treasury said last month that the commission's order to Apple to pay over €13 billion to Ireland was "disappointing" and would "undermine the important spirit of economic partnership between the US and the EU”.

The timing of the new case has prompted speculation that Vestager is trying to appease the US by showing she also takes on European firms.

A commission spokesman denied the claims, saying that the commission "always apply state rules to everybody".

The GDF Suez case does not stem from the LuxLeaks revelations published in 2014 but from a review of tax rulings issued in member states that was launched in 2013.

The review was extended in December 2014 after the LuxLeaks revelations, and about 1,000 tax rulings were examined.

The GDF Suez case is the first opened after the review.

The commission did not say how much taxes it estimates GDF Suez avoided with the tax rulings, nor did it say whether it was the only company in that case in Luxembourg.