A secret test conducted by police showed that terrorists could break into Parliament in under five minutes and stage a massacre of more than 100 MPs, The Sunday Telegraph can reveal.

In a simulated attack, which took place in the middle of the night, officers posing as extremists used a boat to gain access from the river. They then navigated the corridors and stormed the House of Commons chamber in an exercise that could have caused mass casualties if replicated in real life.

The drill took place when Parliament was in recess earlier this year, with the vast majority of MPs and peers being kept in the dark about the results.