BEHIND a balaclava, Matthew MacDonald hid the fact that he was a privileged Eton schoolboy as he joined in the riots that brought mayhem to London on May Day.

But, gloved hand raised in rebellion, MacDonald, 17, also hid a much more sinister secret…he is a member of an anarchist group plotting the downfall of the Royal Family.

And that sparked off a royal security scare because MacDonald was a fellow pupil at Eton of Prince William.

The alarm was raised when he was found carrying a Movement Against the Monarchy membership card when arrested during the May Day riots.

Detectives now fear the teenager could have passed sensitive information about William, 17, and younger brother Harry to the extreme left-wing group.

MacDonald was in a uniquely close position to the young Prince and would have been regarded as a key member of the anarchist group.

He was in the same year as the Prince, would have shared some classroom lessons and was aware of his movements at the pounds 14,000- a-year school.

It is feared that Movement Against the Monarchy (M’AM) leaders could have told MacDonald to gather compromising information about him.

Special Branch officers raided MacDonald’s room at College House, Eton, and his parents’ south London home after discovering his anarchist link.

They took away bundles of personal letters, files and books.

It is now likely that Buckingham Palace will order an urgent review of the security operation surrounding Prince William.

Detectives are also likely to interview members of Eton’s left- wing Orwell Society which was set up by MacDonald. They are also investigating the activities of the 500-strong M’AM which is led by Ian Bone – former head of violent 80s group Class War.

M’AM is planning a mass demonstration outside Buckingham Place on June 3.

Last night, a highly-placed police source said: “The fact that MacDonald is linked to this group has very serious implications for the Palace.

“Guarding the young princes is a highly sensitive and careful operation. If their security has in any way been compromised, then changes will have to be made.

“It is not inconceivable that pupils with anti-monarchy beliefs could be passing on information.”

MacDonald is understood to have become increasingly uneasy about being privileged enough to attend Eton. His father, Professor Theodore MacDonald, said his son had written a letter, resigning from Eton after becoming disillusioned.