LONDON (Reuters) - Four members of an Islamist militant cell who referred to themselves as the “Three Musketeers” were found guilty on Wednesday of plotting an attack in Britain after hashing out their plans on an encrypted messaging service.

Naweed Ali, 28, Khobaib Hussain, 25, and Mohibur Rahman, 33, and Tahir Aziz, 38, were convicted of terrorism offences by a court in London and are due to be sentenced on Thursday.

Ali and Hussain had previously served over three years in prison for terrorism offences after traveling to Pakistan in 2012 for militant training. Rahman had separately been convicted of possessing materials linked to terrorism in 2012.

Police said the men, who had denied the charges, had been planning an Islamic State-inspired attack on British soil.

“These men shared the same radical belief in violent jihad and had reached a stage where they were planning to take action,” Sue Hemming from the Crown Prosecution Service said.

“Recent attacks have demonstrated the kind of horror these defendants could have caused had they not been stopped,” she said.

The four men were arrested in an undercover sting operation in August 2016 after police uncovered a partly-built pipe bomb, an imitation gun, and a meat cleaver scratched with the Arabic word for infidel in Ali’s car.

Authorities also found a samurai sword in Aziz’s car. The discovery of the cleaver gave the case echoes of the killing of Lee Rigby in 2013, when two men used knives and a cleaver in an attempt to behead the soldier.

The court heard that Ali, Hussain and Rahman, from the West Midlands in central England, had joined extremist social media groups and shared related violent material online.

They bought new phones to carry out an attack, police said, and discussed their plans in a group chat on the encrypted messaging app Telegram named the “Three Musketeers” after famous characters in a French historical novel.

Aziz joined the group shortly before they were arrested, the court heard.

Interior minister Amber Rudd has said that encryption is hindering security services from stopping potential plotters, and was in Silicon Valley this week to discuss the issue with tech companies.

The UK’s national terrorism threat level remains at “severe”, which means an attack is highly likely. There were vehicle and knife attacks in London and a suicide bombing in Manchester in recent months.

Officers said the case was “one of the most complex counter terrorism investigations in a number of years” and called for “more to be done” after individuals convicted of terrorism charges are released from prison.