The “resilience test” — which took place while the parliament was in recess — was part of an investigation led by British police | Daniel Leal-Olivas/AFP via Getty Images Mock terrorists took 5 minutes to storm UK parliament in simulated attack If the attack had been real, police believe over 100 MPs could have been killed.

Terrorists could force entry into the Palace of Westminster and storm the House of Commons chamber in under five minutes, U.K. police have concluded following an exercise simulating an attack.

During the exercise, which was carried out during the night earlier this year, officers posing as extremists used a boat to gain access to the building from the river before storming the House of Commons chamber, according to a report in the Sunday Telegraph.

The “resilience test” — which took place while the parliament was in recess — was part of an investigation led by British police five months after a terrorist attack yards from the Palace of Westminster.

In that incident, Khalid Masood forced his way into the parliamentary estate and stabbed a policeman after driving into pedestrians on Westminster Bridge. He was shot dead by police.

The Sunday Telegraph cited sources familiar with the simulated terror attack who said that during the test, it took just five minutes for the attackers to reach the Commons chamber. Police estimated that more than 100 MPs could have been killed, the paper reported.

The newspaper also revealed that an armed guard had now been stationed to protect entrances from the river and that more than 15,000 security passes will be reissued to close security loopholes.

In a separate incident, police detained a man armed with a four-foot sword Friday night outside Buckingham Palace after he attacked three officers. He was subsequently arrested for terrorism offenses. On Sunday, police said a second 30-year-old man was arrested at an address in West London in connection with the incident. He was detained on suspicion of being "involved in the commission, preparation or instigation of terrorism," according to a police statement.