Miliband absent as Nick Clegg, David Cameron and Nigel Farage pose on Sun front page with Help for Heroes wristband

Ed Miliband is locked in another battle with the powerful Sun newspaper after it claimed he had refused to join its campaign to support the charity Help for Heroes, which aids British armed forces veterans.

Nick Clegg, David Cameron and Nigel Farage all posed on the front page of the Sun with a Help for Heroes wristband. However, Miliband’s aides said he was too busy dealing with a shadow cabinet meeting about whether the UK should join attacks on Islamic State (Isis).

According to the newspaper, the Labour party leader turned down four requests to endorse a campaign for Help for Heroes “for fear of offending Labour lefties”.

The Sun’s front page contrasts images of Cameron, Clegg and Farage sporting the wristbands with a white square in place of Miliband.

But a Labour source said: “It’s untrue that Ed refused to back the Help for Heroes campaign. He was busy with the shadow cabinet discussing how to respond to the [Isis] threat in Iraq.”

The Sun said: “We approached him [Miliband] four times in total, most recently yesterday [Wednesday] afternoon, and told him it was going to happen and he would be empty-chaired if he didn’t do it.

“He was not asked to hold up anything Sun-branded. Every day this week there has been a Help for Heroes feature or news story; it has been well flagged up. The other three political leaders posed with selfies.

“The idea that it would have been a big chunk out of his day is just wrong. It was absolutely not a stitch-up: this was the next part of the campaign.”

Miliband came under heavy criticism from his own backbenchers when he posed with a copy of the Sun newspaper two months ago backing England’s doomed drive to win the World Cup. Miliband regretted the decision to pose with the Sun since he had made his political reputation in part by standing up to Rupert Murdoch over phone hacking, and many Merseyside MPs still see the Sun as a hateful publication because of its coverage of the Hillsborough disaster.

Some in Miliband’s office think it is vital the Labour leader courts rightwing newspapers to minimise negative coverage since their readership is critical to election success. Others argue the rightwing press is a lost cause and trying to court it reduces his authenticity.

Some Labour sources said he feared the Sun was trying to embarrass Miliband and using a charity Miliband greatly admired to prosecute a political war against the Labour party.

The Labour leader is also expected to face pressure to condemn the now-admitted phone hacking at Mirror Group newspapers, even though the Daily Mirror has been staunchly loyal to him in the past.