A Ukip MEP has been forced to apologise for apparently calling the head of a Christian charity a paedophile on Twitter.

Jane Collins, one of Ukip’s leading politicians, agreed to make a donation to the charity after she implied that Mark Russell was a criminal on the social networking site on Thursday.

Russell, who is a Labour party member and head of the Church Army charity, had posted support for his party’s candidate in the South Yorkshire police and crime commissioner byelection.

But Collins replied, telling Russell: “Yes because we’d soon stop your criminal activity. Paedos leave our kids alone. #Ukip.”

The byelection was prompted by the resignation of the Labour police commissioner Shaun Wright in the wake of the Rotherham child exploitation scandal.

Collins initially refused to apologise for her paedophile implication, saying: “It was aimed at the paedophiles not you.” But she agreed to delete the tweet because it “read wrong”, explaining: “I’m a bit hot headed sometimes.”

She later added: “I apologise for associating you with paedophiles but you’re still supporting a party that has failed our children badly.”

However, Russell said the Ukip MEP was turning her mistake into party politics and threatened her with legal action. Eventually she conceded: “I am in the wrong … I unreservedly apologise for implication.”

Russell told the Guardian the tweet was highly damaging to his work, which includes supporting vulnerable people. It was made worse because Collins’s accusation was retweeted many times by Ukip supporters, he said.

He told Collins: “I’ve dedicated my life to helping kids and your tweet was as hurtful as offensive … this is my reputation.

“I merely encouraged my fellow South Yorks residents to vote for Alan Billings, the Labour candidate.”

Russell is the head of Church Army, an evangelist charity linked to the Church of England that has the Queen as its patron. He said the MEP had offered to make a donation to the group after he threatened to sue her.

Collins’s tweet follows accusations that Ukip exploited the suffering of Rotherham sex abuse victims for political gain in the police and crime commissioner byelection. One victim denounced the party’s tactics as “disrespectful” after it launched a “1,400 reasons to vote” campaign, in reference to the estimated number of victims in the city between 1997 and 2013.

Collins did not respond to emails from the Guardian.