In a report released yesterday, called Extremism in the Community: The Case of Shakeel Begg, the Henry Jackson Society criticised Citizens UK for allowing Shakeel Begg, an imam at the Lewisham Islamic Centre, to continue working with it after a judge ruled he was an extremist.
Last year Begg lost a libel case against the BBC, which had accused him of being an extremist during a discussion on the Sunday Politics television show.
In that case, the judge ruled that Begg was "an extremist Islamic speaker who espouses extremist Islamic positions".
Despite this, the Henry Jackson Society’s report says, Begg was an official speaker at a vigil for child refugees the charity held outside parliament in December last year.
The Liberal Democrat leader Tim Farron and Stella Creasy, the Labour MP for Walthamstow, also spoke at the event.
The report also criticises Citizens UK for allowing Begg to participate in a number of events in the years prior to the libel ruling.
A statement from Citizens UK said: "The Lewisham Islamic Centre, of which Mr Begg is the imam, has been an active member of our south London chapter for several years. During this time the centre has played a positive role in community cohesion, and worked well with other local institutions, including churches, synagogues and non-faith groups, on various community projects."
The statement said Begg was one of numerous religious leaders who attended the event to lobby MPs to uphold commitments on unaccompanied child refugees.
The statement said: "Lewisham Citizens local leadership seeks to ensure that the activities and moral conduct of members uphold the shared principles of working together to create a better community for local people.
"These leaders continue to be able to work well with the centre to create positive local change and coalesce around issues of mutual concern for the community – many hundreds of whom use the mosque."
A spokesman for the Charity Commission said: "We have been contacted by Citizens UK, which has confirmed that it will be making a serious incident report to us. We will assess the information contained in the report to determine what, if any, regulatory action we may need to take."