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The links of mayoral hopeful Sadiq Khan’s former brother-in-law to one of the UK’s most notorious extremist organisations are revealed today.

Top London lawyer Makbool Javaid was married to the Labour Party candidate’s sister Farhat Khan until 2011.

In the Nineties Mr Javaid took part in events in London with the extremist group Al-Muhajiroun while he was Mr Khan’s brother-in-law, having married the Labour politician’s sister in 1989.

He appeared alongside some of the country’s most notorious hate preachers, including the now banned cleric Omar Bakri, in 1997 and 1998.

Mr Javaid’s name appeared on a fatwa in 1998 calling for a “full-scale war of jihad” against Britain and the US.

Mr Khan attended a conference alongside his then brother-in-law in 2004 but today said that he has had no contact with Mr Javaid for at least 10 years and that he has always condemned extremism.

Al-Muhajiroun — founded in the Eighties by Bakri and another well-known radical London preacher, who cannot be named for legal reasons — became notorious after praising the 9/11 atrocity, the 7/7 bombings and other al Qaeda acts of terror.

It became a banned organisation in 2005. In literature from the Nineties, on which Mr Javaid is described as a “spokesman” for the group, it calls on Muslims to fulfil their “Islamic duty” and launch jihad “physically, financially and verbally”.

One video of a Trafalgar Square rally in 1997 shows him railing against regimes of the “kufr”, a derogatory term for non-Muslims.

He calls for the re-establishment of an Islamic state, berates the West for introducing the “diseases of nationalism, of racism, of secularism” and urges followers to further the faith “until Islam becomes dominant or until they kill me”.

In his animated address at the foot of Nelson’s Column a crowd can be heard shouting “Allahu Akbar”.

His name appeared next to Bakri’s on the fatwa declaring war on the UK and US in 1998, the year he attended another event in Trafalgar Square.

Mr Javaid, 54, now a partner of law firm Simons, Muirhead and Burton, denies ever being a spokesman for the group, says he never authorised his name being included on any of the group’s literature or the fatwa and condemns its contents.

Today he said he “regrets” his Trafalgar Square speech and added that he was “naive” at the time.

The former London School of Economics student now has links with the controversial advocacy group Cage, according to his Facebook page.

He is “friends” on Facebook with its research director Asim Qureshi, who described Islamic State butcher Jihadi John as a “beautiful young man”.

He is also a Facebook “friend” of Cage director Moazzam Begg, a former Guantanamo Bay detainee released without charge following his arrest after travelling to the Middle East.

Mr Javaid says he has not used Facebook for years, adding that it does not reflect his friendships.

In another video unearthed by the Standard, he appeared in a 2012 broadcast by The World This Week — and was described as a “patron” of an organisation called Cageprisoners.

The group, which morphed into Cage, has been linked to the late Anwar al-Awlaki, an al Qaeda figurehead who praised the killing of western civilians. Mr Javaid says he was never a patron of Cageprisoners and appeared on the programme as a human rights lawyer.

He appeared on the 2012 broadcast alongside the brother of Syed Talha Ahsan, from Tooting, who in 2013 pleaded guilty after being extradited to the US for providing material to support terrorism by running a pro-jihadist website from London.

His co-defendent was Babar Ahmad, for whom Mr Khan campaigned to try to stop his extradition to the US before being elected as an MP in 2005. Ahmad later admitted terrorism charges.

Mr Khan attended at least four meetings organised by the Stop Political Terror group campaigning for Ahmad, also a Tooting resident, and spoke outside Woodhill Prison alongside Adnan Siddiqui — now a prominent member of Cage — in 2004. He was also billed as a speaker at two similar events alongside Ahmad’s family in February and March 2005. He shared a platform with Ahmad’s father at an event in Parliament as recently as 2012.

Mr Khan says he supported the campaign for Ahmad to be tried in the UK in his then role as a human rights lawyer and chair of human rights group Liberty.

According to records, Mr Javaid and Ms Khan registered their marriage with Lambeth council in 1989.

However, it is understood the pair divorced in 2011 after separating five years earlier.

Mr Javaid lived with Mr Khan’s sister and their children in a £500,000 semi-detached house in Norbury, south London. He was formerly head of litigation services at the UK Commission for Racial Equality — now the Equality and Human Rights Commission — and has been described by The Legal 500 journal as a “highly respected discrimination law practitioner” who is “business minded”.

Mr Javaid, former chairman of the Society of Muslim Lawyers, was billed as a speaker at several Al-Muhajiroun rallies and his name appeared on several press releases and leaflets for the group during the mid-Nineties.

Some call for the “domination of the world by Islam” and for all Muslims to dismantle western governments and establish an Islamic State, or Khilafah.

Other literature refers to the West as “evil”, berates Israel as a “cancerous entity” and urges Muslims not to vote in elections. Mr Javaid is billed as a speaker at events outside London mosques in the literature.

He claims he did not know at that time that Al-Muhajiroun was a hate group and never authorised his name being included in its literature.

Al-Muhajiroun has for years been linked to a number of extremists — including Lee Rigby’s killer Michael Adebolajo, who attended meetings and demonstrations, and Abdul Waheed Majeed, from Crawley, who became the first British suicide bomber in Syria in February 2014.

Today Mr Javaid said: “Of course I regret the speech I gave and some of the things I said and did in my younger years.

“Twenty years ago I was naive. I certainly didn’t realise how easily some of my actions could be interpreted as being critical of Britain — the best country in the world.

“This was nearly two decades ago now and I have grown and changed. I love Britain, I love the rich diversity of London and I love our culture.”

Mr Khan told the Standard: “I have had no contact with him for more than a decade. I have always condemned the hideous organisations that promote extremism.

“Extremism is a cancer in British society that must be rooted out. I have outlined tough plans to root out extremism and tackle radicalisation as Mayor of London.”

It is understood that the mayoral hopeful attended the 2004 event in his capacity as a lawyer and human rights campaigner.

He did not comment on when he became aware of Mr Javaid’s involvement with Al-Muhajiroun.