By Sudip Kar-Gupta and Foo Yun Chee

BRUSSELS (Reuters) -French President Emmanuel Macron on Tuesday criticised EU antitrust chief Margrethe Vestager's decision to hire a U.S. economist over a European to help oversee Big Tech, adding her previous work could lead to conflicts of interest.

Leaders of the main political groups at the European Parliament have also chided Vestager for picking Fiona Scott Morton, 56, the former chief economist at the U.S. Department of Justice during former President Barack Obama's tenure.

"It implies we have a very serious problem with all the academic systems in Europe," said Macron, as he expressed his astonishment at the fact that a European Union national could not have been found for the job.

Macron added he was "attached to reciprocity", and noted that the U.S. and China would not have appointed an overseas national in such a role.

Scott Morton will advise the European Commission on its investigations into Big Tech and its enforcement of a series of landmark rules to rein in tech giants.

Macron pointed to her previous work for "many companies" as something that could also result in conflicts of interest. "Lots of questions I have to ask myself over this, which leads me to be very doubtful," he said.

Vestager defended her decision at a European Parliament committee hearing, where some French lawmakers berated her for her choice and called for a rethink.

"I find it questionable to assume that someone's nationality will automatically lead to a bias in favor of companies originating from that same nationality," she told a score of lawmakers in the chamber.

Vestager said the Commission had opened the post to non-EU citizens in the search for the best economic adviser. She said previous chief economists at the Commission had also done consulting work without posing any issues.

"If anything, their experience in private companies should be an asset, not an inconvenience. It is common that economists at this level work as consultants in parallel to their academic work," she said.

She said restrictions to avoid conflicts of interest have always been in place for Commission officials, and that Scott Morton would only have to recuse herself in a handful of cases.

Economists including Nobel economics laureate Jean Tirole and 39 others on both sides of the Atlantic have rallied to Scott Morton's defence.

(Reporting by Sudip Kar-Gupta; Editing by Bernadette Baum, Sharon Singleton and Jan Harvey)