A noticeably trim Nick Clegg said he thought he would stand a chance in a fight with Ed Balls because he is taking kickbox lessons in a gym.

The Liberal Democrat leader, who predicted a closely fought contest, spoke of his new strength after he was questioned about reports that Balls and the shadow foreign secretary Douglas Alexander almost came to blows over Europe.

Asked whether he could beat Balls in a fight, the deputy prime minister told Pienaar's Politics on BBC 5 Live: "I'm doing kickbox lessons in a gym to keep myself fit so who knows, if I keep doing that maybe I'll stand a chance. I'm not going to make ringside predictions in such a closely fought contest. By the way I'm rubbish at it but I just do it to try and keep vaguely fit."

Clegg joked about Balls after something of a love-in between the pair last week after the shadow chancellor said he understood why Clegg formed a coalition with the Tories in 2010. The deputy prime minister, who played down talk of a rapprochement with Labour, said he has some political opponents in mind when he dons his new yellow boxing gloves. He said: "I sometimes imagine people who will remain nameless. I love it, it's very good fun. It's very good exercise. It gets stuff out of my system."

Clegg was asked about Balls following a report in the Mail on Sunday – denied by Labour – that Balls and Alexander had to be separated by Dame Tessa Jowell during a row over Europe.

Rachel Reeves, the shadow work and pensions secretary, denied the report. "I haven't seen any fisticuffs in the shadow cabinet with Ed or Douglas or anyone else," Reeves told the Murnaghan programme on Sky News.

• This article was amended on 12 January 2014. The original headline stated that Clegg had said he could beat Balls in a fight; this was changed to more accurately reflect his comments.