The conviction of radical cleric Anjem Choudary shows there is "room for improvement" in UK counter-terrorism legislation, a legal expert has said.
David Anderson QC said the law had "barely touched" Choudary for 20 years, before he was found guilty of inviting support for so-called Islamic State.
The UK must establish whether any "impediments" were making it harder to get convictions, Mr Anderson added.
Choudary, 49, who was arrested in 2014, faces a maximum of 10 years in prison.
Counter-terrorism chiefs have spent almost 20 years trying to bring him to trial, blaming him, and the proscribed organisations which he helped to run, for radicalising young men and women.
The father-of-five - who was arrested in 2014 after pledging allegiance to IS - was convicted alongside confidant Mohammed Mizanur Rahman.
Both men were charged with one offence of inviting support for IS - which is contrary to section 12 of the Terrorism Act 2000 - between 29 June 2014 and 6 March 2015.
The verdict on the two defendants was delivered on 28 July, but was only reported on Tuesday following the conclusion of a separate trial at the Old Bailey.
Home Secretary Amber Rudd said Choudary and Rahman had "poisoned the minds of vulnerable people and their warped and twisted propaganda offered support and succour to a murderous and barbaric terrorist organisation".
She said the government would continue to confront those who "promote hate and threaten our way of life", to protect communities from extremism, and defeat "this toxic ideology".
Mr Anderson, the government's independent reviewer of terrorism legislation, said there were "useful offences" for prosecutors to use to charge people spreading radical ideas.
However, he told BBC Radio 4's Today programme these had not helped authorities bring Choudary - one of the UK's most notorious radical clerics - to justice more quickly.
"In the meantime a lot of people have been radicalised," Mr Anderson said, adding: "We do need to look at what might be done if there are impediments, technical reasons why it's not as easy to get convictions under these laws as it should be."
He said it was "very difficult to craft a law that can clearly distinguish people who are dangerous from people who are simply revolting".
He added: "If you go back 10 or 12 years you had home secretaries saying the criminal justice system is broken, it couldn't deal with terrorism.
"I think we've shown now that that is wrong but of course there is room for improvement."
Preventing the continued influence of Choudary while he was in prison could be a "very difficult job", added Raffaello Pantucci, from the Royal United Services Institute.
He told Today that Choudary was a "bright flame" who had drawn people to him, but that finding himself "sitting in a prison cell will mean that his network will be degraded".
However, he added that Choudary was "a very charming and charismatic individual and undoubtedly if he's in prison he's going to be surrounded by very vulnerable and impressionable people".
"I would not be surprised if some of it may rub off on them," he said.
Police have said Choudary stayed "just within the law" for years, but that many people tried for serious terror offences were influenced by his lectures and speeches.
During his trail, the jury heard how Choudary and Rahman invited others to support the militant group through speeches and announced their own oath of allegiance to its leader.
The oath of allegiance had been a "turning point" and meant they could be put on trial, said the Metropolitan Police.
Curry house meeting
When IS announced a "Khilafah" - an Islamic state - in June 2014, the court heard that Choudary held a meeting with his closest aides at a curry house in east London.
Before accepting the "Khilafah" was legitimate, the jury heard he consulted his "spiritual guide" Omar Bakri Mohammed, who is currently in jail in Lebanon.
On 7 July 2014, the men's names appeared alongside Rahman's on the oath, which stated the al-Muhajiroun had "affirmed" the legitimacy of the "proclaimed Islamic Caliphate State".
Commander Dean Haydon, head of the Met's counter-terrorism unit, said the conviction of Choudary and Rahman was a "significant prosecution in our fight against terrorism".
Sue Hemming, head of counter-terrorism at the Crown Prosecution Service, said the two men "knowingly sought to legitimise a terrorist organisation and encouraged others to support it".
"They used the power of social media to attempt to influence those who are susceptible to these types of messages, which might include the young or vulnerable," she added.
The two men will be sentenced in September.