By Dominic Casciani, BBC News

It has been a long time coming - but one of the most hardline Islamist groups ever to have organised in the UK is being banned.

For years al-Muhajiroun in various forms has set out to provoke a reaction from other Muslim groups, politicians and the media.

After months of testing the Home Office with increasingly provocative pronouncements on its website and on the streets, the ban has come. The question is what practical effect will it have?

A ban might look symbolic - but it does have some weight to it. Under the law, anyone who is found to be a member of al-Muhajiroun, which is also known as Islam4UK, will face up to 10 years in jail.

Anyone who supports the organisation, such as in a street rally, could also face the same charge. The law also covers the wearing of a uniform, because it was originally devised to cover Northern Ireland's paramilitaries.

If the police identify any financial assets, they will be seized - and its website will be shut down.

But the difficult question to answer is whether the law has any effect in stopping people from organising.

Republicans would organise honour guards of "volunteers" to carry coffins of their dead

If the police and security services are monitoring meetings involving former members of al-Muhajiroun, any charge will need to prove in court the individuals had all knowingly met as members of the group.

Of the 60 organisations proscribed under terrorism legislation in the UK, 14 of them are from Northern Ireland.

The most important and largest of these is the IRA. When membership became an offence, an organisation like the IRA was never going to simply break up because Parliament had banned it.

Instead, its members stopped organising in ways that would leave them open to a charge.

During the Troubles, this led to the legally curious situation where Republicans would organise honour guards of "volunteers" to carry coffins of their dead. They wore military fatigues, black berets and dark glasses.

There was nothing to officially say this was an IRA funeral - but everybody knew it was.

So what a ban comes down to is a situation where the government is trying to make it harder for a group to organise openly; proscription is just one weapon in the security armoury.

Ban challenged

The only group to have challenged a ban and won is a small Iranian nationalist group, the People's Mujahedeen Organisation of Iran.

PROSCRIBED GROUPS IN THE UNITED KINGDOM Sixty groups are banned under Terrorism Act 2000 Half are international Islamist organisations Fourteen are from Northern Ireland One group has won a challenge Members risk 10 years in jail

Its supporters in the UK fought a long campaign to be taken off the terror list. What was clear at the time was the group's supporters did not stop meeting just because the organisation was banned.

In fact, many of the members used the ban as a means of promoting their cause more widely. Eventually, they had a group of parliamentarians behind them willing to question the Home Office's decision.

Omar Bakri Mohammad, founder of al-Muhajiroun, has already sought to capitalise on the ban. He told the BBC: "[The ban] will increase the popularity of al-Muhajiroun and increase the membership, and I think it is a grave mistake because it will force them underground and [Home Secretary Alan Johnson] is playing with fire.

"I think that it's really a grave mistake because they will realise this war is against Islam in general and Muslim youths in particular."

Inayat Bunglawala, of the Muslim Council of Britain, said:

"There's a risk [that young Muslims will be radicalised]. We have already seen Anjem Choudary [the leader of al-Muhajiroun] saying, 'look, that the government has had to resort to banning our group shows that democracy is just a charade'.

"What we should have been doing as a confident democracy is actually upholding those values of pluralism, that we can tolerate people whose views are so outlandish, so repulsive that providing they do not step over the line and break the laws, we will tolerate them."

Breaking the law, rather than repulsive views, is at the heart of the ban, say officials.

The government is at pains to say the ban was not prompted by al-Muhajiroun's planned protest in Wootton Bassett, Wiltshire, to honour Muslims killed in the Afghanistan conflict - although officials concede the timing does not look great.

Secret analysis

But behind the scenes, the wheels of the security system have been turning. The BBC understands the Joint Terrorism Analysis Centre (JATC) has been leading the review of al-Muhajiroun ever since it declared it was "re-forming" earlier in 2009.

Al-Muhajiroun described the 9/11 hijackers as the "Magnificent 19"

This team within MI5 collates information and intelligence on threats, with the involvement of 16 government departments.

JTAC's report was handed to the home secretary last week - and its secret contents led him to conclude a ban was necessary.

A group can be banned for planning acts of terrorism - but also for glorifying it. Al-Muhajiroun fall into the latter category because of its track record of celebrating acts of violence, including once describing the 9/11 hijackers as the "Magnificent 19".

We do not know what the group will do next - but it does have the right to challenge the ban. It is perhaps worth looking at what al-Muhajiroun has done in the past. It officially disbanded in 2004 - only to reappear under different names a year later.

Omar Bakri Mohammad remains banned from the UK - but his regular internet broadcasts from Lebanon are easy to find and watch.

And Inayat Bunglawala warned: "This ban has been tried before. Al Ghurabaa and the Saved Sect [two of al-Muhajiroun's related organisations] were also banned. All that happened was that the same faces reappeared under different names."