A secret test showed terrorists could break into the British 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, police 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 a move 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.

But three sources familiar with the incident have given accounts to this newspaper after being shocked by the security weaknesses it revealed. One said the public would be "horrified" if they knew what had happened, while another warned: "We are sitting ducks."

The dramatic outcome of the "resilience test" can be revealed as part of an investigation into Parliament's security five months after the Westminster terror attack. It can also be disclosed that:

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• An official reviewer has recommended installing a barrier in the river to stop boats approaching Parliament;
• An armed guard has now been tasked with protecting entrances from the river to stop anyone gaining access illegally;
• A stretch of perimeter fencing just over 3ft high has been flagged as a security risk amid fears terrorists could jump over it;
• More than 15,000 security passes will be reissued after officials accepted the current system had loopholes.

Security around London's landmarks was back in the spotlight this weekend after a man was arrested brandishing a 4ft sword outside Buckingham Palace.

The 26-year-old, who caused minor injuries to three police officers, was being questioned by counter-terrorism police last night.

In March, the jihadist Khalid Masood managed to enter the parliamentary estate through an open gate after running over pedestrians on Westminster Bridge. Masood fatally stabbed Pc Keith Palmer, an unarmed officer, and got around a dozen meters into a courtyard behind the security perimeter before being shot. The attack raised
serious questions about Parliament's security, with footage emerging of Theresa May being scrambled into a car and a motorbike entering the grounds unchecked in the aftermath.

But it was not the first time vulnerabilities had been laid bare. This newspaper has learnt that the remarkably lifelike "resilience test" carried out just weeks earlier had exposed weaknesses.

Dozens of police officers and volunteers were placed in the Palace of Westminster at night to recreate the security conditions during the day. The test was conducted while Parliament was on recess, meaning MPs and peers were unlikely to be around - minimising the chance of it becoming public.

A group acting as terrorists approached Parliament by boat. They gained access via a terrace before storming the Commons Chamber, where debates are held. The time between starting the "attack" and entering the Chamber was less than five minutes, sources said. Simulations showed more than 100 MPs could have been killed.

Those briefed on the drill were left shocked. One source said: "I remember thinking 'Jesus Christ, if that's where we are at and that can happen, then the public would be horrified."

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Another said: "They [the public] would be scared stiff. They wouldn't come back to the place. It is not safe. We are sitting ducks in Parliament."

Measures have already been taken to improve security, with an armed officer now guarding the river entrances. Further changes have been recommended.

A spokesman for the Houses of Parliament said: "The security of Members, staff and the visiting public is our highest priority. While we cannot comment on the specifics of our security, we work closely with the police, security services and others to ensure our security measures are effective and meet whatever level of risk Parliament faces. These measures are always under review."