Neelie Kroes attends a news conference on the European Commission in Brussels September 1, 2014. REUTERS/Laurent Dubrule

BRUSSELS (Reuters) - The European Commission rejected criticism from one of its own former antitrust chiefs on Friday of its massive tax demand on Apple and pointedly noted that she now works for another U.S. tech company, Uber.

Neelie Kroes, who was EU competition commissioner for five years until 2010 and ran digital policy until two years ago, said this week’s ruling by current antitrust chief Margrethe Vestager that the iPhone maker must pay 13 billion euros of back taxes that Ireland had waived was an incorrect use of EU law.

“EU member states have a sovereign right to determine their own tax laws. State aid cannot be used to rewrite those rules,” Kroes wrote in the Guardian. “However, the current state aid investigations into tax rulings appear to do exactly that.”

Asked about the article at a regular news briefing on Friday, chief Commission spokesman Margaritis Schinas said:

“We understand that it may be sometimes challenging to reconcile the role as a former commissioner with the temptation to publicly express the views of those in Silicon Valley or elsewhere who oppose the Commission’s decisions.”

Kroes, a Dutch liberal who left Brussels when Jean-Claude Juncker replaced Jose Manuel Barroso as Commission president in 2014, now sits on the public policy board of Uber. The Silicon Valley company that runs the ride-hailing app is lobbying the Commission to do more to challenge national rules across the Union that have curbed Uber’s development in Europe.

Schinas said Kroes’s criticism of Vestager’s use of EU laws that ban governments giving firms unfair “state aid” -- in Apple’s case via a tax deal -- was incorrect: “State aid rules ... were not applied in the way it was alleged,” he said.