And that the empty property was in Dagenham, under the watchful eyes of Mr. Harris.

“I almost fell out of my chair,” Mr. Harris said of the moment he read of the prayer room in the planning application. “There would be room for thousands of Muslims.”

He launched a campaign against the project, with one rally at a local pub and another where one of his supporters waved a copy of the Quran to show what they were fighting. At one point, Mr. Harris called Mr. Siddique to accuse him of trying to build a new mosque in Dagenham.

“You have turned it into a political football,” Mr. Siddique responded, and withdrew the offer.

The factory grounds are still empty, a point of pride for Mr. Harris. On a recent day, he steered his white Kia past squat rowhouses and shuttered stores in Dagenham in a tour of his resistance to what he calls “the Muslim plan” for a “huge march of mosques.” He passed two Muslim community centers that he suspected, without basis, of hiding secret houses of worship, as well as a long-closed pub where a Muslim entrepreneur had opened a banquet hall that he found suspicious.

At 51, Mr. Harris owns a service station and leads the local branch of the U.K. Independence Party, or UKIP, which helped drive the Brexit campaign. Nativism has a long history in Barking and Dagenham. Neo-Nazi skinhead gangs roamed its streets in the 1970s, and in local elections in the 2000s the far-right British National Party won about 20 percent of the vote.