Membership or having links to the Islamist organisation, which is an outlawed opposition force in Egypt and has a base in London, should be considered a "possible indicator of extremism", Cameron said.
His written statement contained the main findings of a review into the Muslim Brotherhood's activities whose publication has been delayed for months.
Britain has come under pressure from countries like Egypt and Saudi Arabia to ban the Muslim Brotherhood, and ministers had been accused of delaying the review so as not to upset allies in the Middle East.
Egypt has cracked down on the Islamist group since its leader Mohammed Morsi was ousted as president by the military in 2013, with thousands of supporters arrested and hundreds condemned to death in mass trials.
Although the report stopped short of a ban, Egypt welcomed it as "an important and serious step taken by Britain in the path of combatting extremist thought and terrorism".
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"The results of the report confirm that there is a greater realisation at the international level of the extremist and violent nature of the Brotherhood," Egyptian foreign ministry spokesman Ahmed Abu Zeid said in a statement.
Cameron said the government would keep under review "whether the views and activities of the Muslim Brotherhood meet the legal test for proscription".
"Parts of the Muslim Brotherhood have a highly ambiguous relationship with violent extremism," the prime minister said, describing the group as "deliberately opaque and habitually secretive".
The Muslim Brotherhood rejected the review, saying it had not been given a chance to respond to any criticism and raising the question whether "the review was undertaken as a result of pressure by non-democratic regimes".
"We do not accept that these conclusions can be based on credible evidence," said the head of its foreign relations desk, Yehia Hamed.
"It is now clear that the Prime Minister did not intend to carry out a fair review into the Muslim Brotherhood."
A London-based lawyer for the organisation said: "We will be challenging the process of the review and its findings in court".