A lawyer has been convicted of assaulting a fashion designer during a dispute over a front-row seat at an opera performance.

Matthew Feargrieve was found guilty in London of punching Ulrich Engler at least once while attending a performance of Wagner at the Royal Opera House on 7 October last year.

Feargrieve, 43, attacked Ulrich Engler, who had climbed into an empty seat next to him and moved Feargrieve’s partner’s coat at the Royal Opera House in central London.

Engler was attending a performance of the Ring Cycle by Wagner last year when he moved into the vacant seat, next to Feargrieve and his partner, Catherine Chandler, before the beginning of the performance.

The fashion designer told City of London magistrates court he had moved into the chair in row A from his seat in row B at the opera three days earlier before the second instalment of Wagner’s four-part epic, and did so again on 7 October.

Matthew Feargrieve, 43, leaves City of London magistrates court. Photograph: David Mirzoeff/PA

Engler said he asked Chandler if he could sit in the vacant seat next to her. When she said he could not, he asked whether she had purchased the extra seat.

Chandler said she had not, so Engler moved into the seat with a better view than his own, moving her coat and putting it on her lap.

Feargrieve then “lashed out” and punched Engler at least once. Engler, whose clients include the Countess of Derby, told the court he was attacked at least 10 minutes into the performance and left with injuries to his left side.

“I had never seen someone looking with so much anger and terror at me,” Engler said. “He was up, leaning over the woman hitting me. While Mr Feargrieve was hitting me he said, ‘How dare you talk to my wife like this?’

“The only three things I said to this woman was, would you mind if I sat next to you again, have you paid for the seat, and that I was sorry.”

The court heard Chandler tried to calm Feargrieve and the couple left. Engler was subsequently arrested at the opera house on suspicion of causing grievous bodily harm but he was not charged.

Engler was banned from the Royal Opera House after the incident.

Ulrich Engler. Photograph: David Mirzoeff/PA

Elaine McMaster, a former physiotherapist who was attending the opera, told the court Engler “neatly” and quietly climbed over without causing a disturbance to Feargrieve or Chandler.

However, she criticised the couple for their conduct at a previous performance.

“They spoiled the whole evening for me, I couldn’t see the stage at all,” she said. “They just didn’t seem at all concerned by the fact they were annoying everyone in their area.”

McMaster said the couple were distracting other patrons by swaying side to side to the music as Chandler was flicking her hair and being noisy. “People don’t behave like that in their local cinema, let alone in the Royal Opera House,” she added.

Giving evidence, Feargrieve said he only pushed Engler and did so in self-defence after Engler “attacked” Chandler, his partner of 13 years.

He claimed that his shoulder was dislocated as a result of Chandler being shoved into him by Engler. District Judge John Zani did not accept Feargrieve’s evidence or argument of self-defence and, after a one-day trial, the lawyer was found guilty of common assault.

Zani said: “I was not in the Royal Opera House on the day in question so I have to decide from the evidence I have heard and read.

“The events that unfolded occurred in the way Mr Engler recalled. I am entirely satisfied that the prosecution have dismantled the defence you have put forward. This is not a case of one’s man punch is another man’s shove.”

Zani added: “Your evidence has been consistently that maybe fingertips touched. I accept the evidence of the prosecution witnesses where they say you punched Mr Engler at least once maybe twice or three times. I am entirely satisfied that you are guilty.”

The court heard Engler was diagnosed with PTSD after the incident.

Feargrieve, of Barnes, south-west London, will be sentenced on 22 January at Westminster magistrates court. Zani said he was unlikely to face a custodial sentence.