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.

“We suspect they were just wanting to crow to the Kremlin that they had infiltrated the British parliament. It’s not ideal but they were caught before any damage was done,” a parliamentary source said.