In Saturday's attack, three men, wearing fake suicide vests, mowed down pedestrians on London Bridge in a van, before slashing and stabbing revellers in Borough Market, a bustling district of late-night bars and restaurants.
The Metropolitan Police says the body was found on Tuesday downstream near Limehouse from the bridge.
Thomas, 45, has been missing since Saturday.
"We are paying a heavy cost in these attacks", Macron said.
If confirmed, Thomas would be the eighth person killed in the vehicle and knife attack. Nearly 50 were wounded.
The trio killed eight and injured 48 in their rampage around London Bridge and Borough Market on Saturday night.
The 30-year-old was held on suspicion of the commission, preparation or instigation of terrorist acts during a search of a property in Ilford and taken for questioning at a south London police station, Scotland Yard said.
Ms Delcros is said to have been seriously injured.
Third London Bridge Attacker Identified
People look at a floral tribute near London Bridge, London , on Wednesday, commemorating the victims of Saturday's attack. London Mayor Sadiq Khan said questions would need to be asked about what the police knew about Butt.
Xavier Thomas is seen in an undated handout photo.
Pakistan-born Khuram Butt, 27, and Rachid Radouane, 30, both from Barking, east London, were the other two attackers. We have had three horrific attacks and we have foiled five others.
London police have identified the three assailants in Saturday night's terror attack, that killed seven people and injured almost 50 more, as Khuram Shazad Butt, 27, Rachid Redouane, 30, and Youssef Zaghba, 22. Zaghba was "not a police or MI5 subject of interest", it added, an assertion that seemed to conflict with accounts in the Italian media.
The Corriere della Sera newspaper said Italian intelligence had made Zaghba's presence and movements known to the British authorities.
An official at the Bologna chief prosecutor's office said Zaghba was stopped at the city's airport after arriving on a flight from London 2016.
Italian sources immediately revealed that the 22-year-old had been on their watch list since his arrest in March last year trying to board a one-way flight to Turkey. But he successfully got them back after a court determined there wasn't enough evidence to arrest him. "But I did my best, and I think he was worn down on the inside", she said. Zaghba spent a lot of time with relatives in East London but visited his mother several times in Bologna.
Butt was the office manager according to a statement from Auriga Holdings, the holding company.
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