A dramatic night-time operation featuring police posing as terrorists has exposed security weaknesses at the Palace of Westminster, according to reports.
The Sunday Telegraph said a simulated attack on the palace was conducted earlier this year when Parliament was in recess, with officers using a boat to gain access to the Thames-side building from the river.
A spokesman for the Houses of Parliament authorities would not confirm or deny that the operation had taken place.
But the paper said that three separate sources had given accounts of what had happened, with one saying it showed that MPs were "sitting ducks".
The officers were reported to have stormed into the Commons chamber within five minutes in a way which could have led to a "massacre" if the House had been sitting.
A review of security at the House of Commons is under way following the knife attack in March which killed a police officer guarding the palace's front gates.
The Telegraph said the review had highlighted the need for a barrier in the river to stop boats approaching Parliament, as well as an armed guard to protect entrances from the Thames.
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 that our security measures are effective and meet whatever level of security risk Parliament faces.
"These measures are always, and will continue to be, under constant review."