U.S. Director of National Intelligence James Clapper warned Tuesday that Brussels-style Islamic State terror cells are operating in England.

Clapper told reporters at an event hosted by the Christian Science Monitor that ISIS militants have used Europe’s open borders and the refugee crisis to infiltrate the continent. He said there was a “fundamental conflict” between national security and free movement throughout the European Union.

He said there were also similar terror cells operating in Germany and Italy.

“We continue to see evidence of plotting on the part of ISIL in the countries you name. They have ten advantage to some extent of the migrant crisis in Europe, something which nations have a growing awareness of,” Clapper added.

The Telegraph reported that Clapper’s comments have echoed warnings issued by the head of Britain’s national security that the Islamic State intends to carry out mass casualty attacks throughout Britain.

The paper noted that Clapper was the most senior Western official to acknowledge publicly the terror group’s extensive reach into the continent. He also said months after the attacks, a portrait of the extremists’ presence in Europe began to emerge.

Chairman of the commons home affairs select committee Keith Vaz said Britain needs to take Clapper’s warnings very seriously because his message comes from a “key ally.”

“There are worrying concerns about free movement across the EU. Once someone passes through the external border of the EU there are simply no checks before they get to the UK,” Vaz said. “Even there they are not challenged robustly enough as they are EU citizens. These gaps need to be addressed.”

More than 800 people are believed to have traveled to Iraq and Syria to join ISIS and more than half are believed to have returned to Britain, according to The Telegraph.

Mohamed Abrini, a key suspect in the Paris and Brussels attacks, is believed to have traveled and met with contacts in Britain last summer. At least five people were arrested in Birmingham and Gatwick earlier this month as part of wide scale anti-terror raids.

Clapper said that better intelligence sharing between European countries was an important issue that needed to be addressed to prevent future attacks.

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