When I saw the face of Khuram Butt staring out of the newspapers at me last week, after he was named as one of the three London Bridge killers, I was deeply shocked.

It took me back to the day nearly two years ago when I filmed him with a hidden camera as he protested, clad in headdress and robe, outside Regent's Park Mosque in London.

We had gone undercover to make a Dispatches documentary for Channel 4, focusing not on Butt and his fellow Islamists but on a less obvious group – the 'sisters', the women who share their twisted views. Unlike their male counterparts, the women only ever express these views behind closed doors or online.

Now my greatest fear is that one of those women I watched being brainwashed – or, even worse, one of the children who ran around playing as the hate-filled rants were delivered – might go on to murder innocent people as Butt did.

After months attempting to win their confidence, I was introduced to Rubana (pictured), who used the pseudonym 'Umm Luqman', and was invited to one of her talks in a nondescript building next to a barber's shop in Whitechapel, East London

A Channel 4 programme has exposed a group of British women as ISIS supporters who are encouraging 'impressionable young girls' in the UK to travel to Syria and join the terrorist group

I didn't pay much attention to him that day in July 2015 as he handed out extremist leaflets to worshippers leaving Friday prayers, but last week's events made me recall the vile propaganda he was peddling – in the same manner as the women, as I would discover.

They were part of banned terror group Al-Muhajiroun in all but name. Its former leader, Anjem Choudary, had just been arrested, and is now serving a prison sentence for urging support of Islamic State.

But his followers continued their 'work' without him – and no doubt still do now. What's more, the woman leading the 'sisters', is Choudary's wife, Rubana Akhtar.

Over the course of a year, I slowly gained access to Rubana's group, initially by contacting her followers on Twitter, then through texting and other social media.

After months attempting to win their confidence, I was introduced to Rubana, who used the pseudonym 'Umm Luqman', and was invited to one of her talks in a nondescript building next to a barber's shop in Whitechapel, East London.

What I witnessed there convinced me that the stereotypical image of jihadis as young men needs to be adjusted, because the women were equally virulent in their views.

In the two-hour lectures, the female Islamic State sympathisers are heard spreading ISIS's extremist ideology and encouraging people to join them in Syria. Pictured is a still from the documentary

ISIS ideology was spread both online and in person to women and 'impressionable young girls'. They are told to abandon democracy and travel to Syria to join ISIS in a programme that aired on Channel 4 in 2015

An undercover reporter was able to meet them after extensive direct messaging on Twitter. One of the women, identifying herself as Umm Usmaan, wrote on Twitter a call for young Muslim women to travel to Syria

The documentary followed a 12-month investigation which saw a team of reporters infiltrate the group's inner circle

The hatred toward non-believers runs deep in this politicised version of Islam. The women believe that the self-declared Islamic State is the true caliphate. In essence, they are swearing their allegiance to IS, which has declared a war on British and Western society.

The casual way this small but motivated group trot out such abhorrent ideas in the presence of small children shows just how 'normal' they believe them to be.

I always felt the group were suspicious of me – and that was to prove the case. As a result, I believe they toned down their rhetoric in my presence but that didn't prevent them from espousing truly shocking views which I captured on film.

Rubana referred to 'filthy Jews' and praised the advent of IS, saying 'The good days have already begun, nobody would ever have thought in our lifetime we would see the establishment of the Khilafah [IS].'

She also mocked the Government's attempts to tackle extremism, saying: 'If they thought it was a plan to deradicalise people, God they got it so wrong, because if anything more and more people are becoming what they call radicalised.'

The British Women Supporters Unveiled sees three women exposed as ISIS sympathisers. (pictured is a still from the programme)

When I saw the face of Khuram Butt (pictured) staring out of the newspapers at me last week, after he was named as one of the three London Bridge killers, I was deeply shocked

She even laughed about the Jordanian pilot who was burned alive by IS, suggesting that he would be accepted in martyrdom, before adding 'My foot', as her followers chuckled.

There may be a temptation to write these people off as harmless idiots, but let's not forget that Al-Muhajiroun have been linked to half of all UK terror plots and, although they have been a proscribed organisation since 2010, it is clear from what I saw that they still exist. We urgently need to address what extremism is, and what to do about it.

Perhaps I asked too many questions while undercover, because eventually Rubana cornered me in a small kitchen area as one of her followers stood behind me. She accused me of being a spy and demanded to look in my bag. I was scared they weren't going to let me go, because she said she wouldn't.

The stand-off went on for about 10 minutes. At that stage, I'd just discovered that Rubana was married to Choudary and that made her appear more dangerous. There were so many of them, there was nothing I could have done to protect myself if they had turned on me.

In the end, I pretended I was mentally ill and went into an emotional meltdown, screaming at her, which seemed to work, and thank goodness they let me go.

Looking back, I think I was lucky to get out unharmed.