Membership or association with the Muslim Brotherhood is a "possible indicator of extremism" but it will not be banned, the prime minister has said.
A long-delayed review of the international Islamist network found parts of it had an "ambiguous relationship with violent extremism".
David Cameron told MPs the group's activities would be kept under review.
The group's British lawyers say the Brotherhood will legally challenge the "deeply flawed" findings.
In a statement, the group said the UK's position was unjust.
The Muslim Brotherhood has many different branches and affiliated organisations across the world - and remains banned in some parts of the Middle East.
Some groups proscribed under terrorism laws in the UK adopted the Brotherhood's core thinking about a global Muslim identity into their own ideologies. [BBC] Read more
