Exclusive: MPs warned not to invite Russian diplomats to parliament after incident said to have alarmed ministers

Russian diplomats accessed a private area of parliament in a major security breach just before Christmas that has alarmed security officials and prompted private warnings from the speakers of both houses, the Guardian understands.

The small group of diplomats joined a public tour of the Houses of Parliament and then broke off to enter a part of the House of Lords that was out of bounds, before they were found by security and thrown out.