"I think they are a serious rival for us," Sagna, who plays for Manchester City, told reporters ahead of the host nation's opening game against Romania on Friday.
"Like us it's an extremely young team and it has a certain quality going forward. In my opinion, it's the most serious English team in a long time."
Warm-up wins over Turkey, Australia and Portugal have raised hopes of Roy Hodgson's charges ending the country's 50-year wait for a major international trophy since England's World Cup triumph in 1966.
Speaking before Sagna, France's number two goalkeeper Steve Mandanda said it was an "obligation" for his country to win the competition, but that Les Bleus should be wary of taking Romania too lightly in their opening game at the Stade de France.
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"The pressure is on us. We know Romania don't have big names, but they are players who have done well finishing second of their group with the best defence of all the qualifying teams. It'll be difficult for us and we will respect them."
(Reporting By John Irish; editing by Martyn Herman)
