Nigel Farage has become embroiled in a potentially damaging internal row over allegations that Ukip's leadership has failed to act on serious matters of governance, including taking steps to dissociate the party from a convicted paedophile.

The incendiary accusations have seen a former prospective Ukip MEP resign from the party and led to toxic claims that the party lacks a robust system for examining internal complaints.

But a Ukip spokeswoman has said the claims had no foundation, had been investigated, and were the views of a disgruntled member who had failed to get the party to back his candidacy.

In a letter to Farage, obtained by the Observer, David Gale, interim chief executive of Kids for Cash UK, a charity that campaigns against paedophiles, said he had no choice but to resign from the party after becoming frustrated that his fears about a registered sex offender and Ukip supporter were not being taken seriously. The issue was also raised by other members of Ukip, according to emails sent to its chairman, Steve Crowther, seen by the Observer.

"The party's response has been absolutely extraordinary," Gale said. "They've done nothing. This is another example of Ukip's structural weaknesses." Gale suggested the party had chosen to close ranks rather than examine his allegations. In claims fiercely denied by the party, Gale warns Farage: "There is a core faction associated with the party that is being used as a 'Black Ops' dirty tricks team against targets that include party members."

He tells Farage: "I believe that your own deep insecurity and paranoia have played a significant part in the party's Night of the Long Knives" – a reference to a recent purge when a number of party members were asked to leave Ukip.

Gale, who stood as a Ukip police and crime commissioner, claimed there was evidence social media were being manipulated by people close to the party.

"There is increasing evidence that points to Ukip members close to the party's headquarters being involved in the creation of cloned Facebook pages which have featured damaging fake posts falsely attributed to Ukip members," Gale writes. "Screen grabs of these posts have been fed to the press, third hand, in an attempt to smear the targeted individuals and as a precursor to removing them from the party. These are potentially criminal acts."

The accusations have been bandied back and forth in recent weeks, triggering an internal debate among Ukip's members about the party's direction. A number are concerned that Ukip has become too centralised under Farage and that Crowther has failed to act on serious allegations involving fraud, claims denied by the party.

"I have been raising the issue of the lack of scrutiny and governance within the party's management," Gale tells Farage. "One of the starkest examples of this lack of governance has been the tolerance of the party's continued association with a convicted paedophile who, whilst no longer being a member, has continued to attend party functions and provide strategic Ukip documentation from his personal website."

Gale said he had supplied evidence of the registered sex offender's continued involvement with Ukip to the party. The man, whose name is known to the Observer, has campaigned on its behalf and blogs on party issues. He was placed on the sex offenders register for life in 1999, having served a 30-month sentence for offences involving children.

Gale told Farage: "My efforts to raise the alarm have been ignored by the leadership, it being left to me to contact branches to alert them to potential risks to members and to have photographs of the paedophile campaigning for Ukip removed from branch websites."

A Ukip spokeswoman said Gale's complaints had been investigated fully. "These are entirely bogus allegations," she said. "They represent a deliberate attempt to cause trouble for the party by someone on his departure."

However, the row raises questions over Farage's leadership. Considered to be the party's greatest asset, there are questions among Ukip grassroots members about whether Farage is focused on gaining a seat in the House of Lords.

"It's no secret Farage is focused on a peerage," Gale said.