A 'war game' that simulated gun-wielding terrorists ramming a car through the gates of Parliament is said to have exposed glaring weaknesses in security.

The exercise is said to have found that four extremists armed with automatic weapons could get into the Commons chamber - and ended with 'most MPs dead'.

The grim scenario was apparently tested by the security services and parliamentary authorities over the past year.

It underlines concerns about weaknesses in the defences at the heart of British democracy - after Khalid Masood was able to maraud on to the estate and Pc Keith Palmer to death, having mowed down dozens of pedestrians on Westminster Bridge.

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Khalid Masood used a hired car to mow down dozens of pedestrians on Westminster Bridge before entering the grounds of Parliament and stabbing Pc Keith Palmer to death

There are questions about why there were no armed police manning the main Carriage Gates at parliament, which were open to allow ministers to drive in and vote. He was only shot by Defence Secretary Michael Fallon's close protection officers.

It has also emerged the gates remained open and unprotected for several minutes after Masood's deadly assault.

MailOnline revealed on Friday that MPs had railed against armed police being deployed at the Parliamentary gates because they created a bad 'atmosphere'.

There were also a slew of complaints that security barriers to prevent terrorists ramming cars into the building were 'ugly'.

The complaints, in response to a survey by the House authorities in 2014, lay bare the extent of resistance among politicians to modernising security on the estate.

There is now pressure for the area outside the palace to be pedestrianised - reviving a long-standing project that has been championed by Sadiq Khan. Barring traffic from the area would potentially make access easier to control.

Scotland Yard's anti-terror chief Mark Rowley has insisted that Parliament's security arrangements were 'proportionate' - but confirmed they would be reviewed.

The Carriage Gates at parliament have been identified as a 'weak spot' in its defences

Critics have claimed the main entrance to the Palace of Westminster had long been a weak spot on the estate because the imposing iron gates are left open for most of the day.

According to the Sunday Times, a 'war game' staged by police commanders, the parliamentary authorities and security experts considered what would happen if four terrorists with automatic weapons broke through the gates in a 4x4 vehicle.

The 'table-top' exercise concluded that the gunmen were able to enter the Commons during a vote and 'most of the MPs died'.

Lib Dem MP Tom Brake, a member of the ruling Commons Commission, suggested on Friday that officers manning the gates should always be armed or accompanied more closely by armed colleagues.

SECURITY WAS BEEFED UP AFTER 9/11 AND TUBE BOMBINGS The £30million a year security operation at parliament has undergone a massive overhaul in recent years. As the terror threat increased in the wake of 9/11 and then the London bombings, there were increasing concerns that the protection was too weak and ad hoc. Barricades were installed outside to prevent vehicles being rammed though the walls. Passes to gain access to the estate were made more secure. A glass screen was also installed in the public gallery after Tony Blair was targeted with flour by protesters. Traditionally the Serjeant-at-Arms has been in charge of security on the Commons estate, assisted by a team of so-called 'men in tights'. Black Rod has an equivalent role in the Lords. But the officials have not been obliged to have any special security background - typically being promoted from within the ranks of the Parliamentary service. In practice, much of the protection for the Palace has been provided by Scotland Yard through a contract with the Parliament authorities. Around 500 personnel are thought to have been involved. In 2011 the Houses finally decided that the arrangements were not coherent enough, and created a post of Parliamentary Security Director with a dedicated team to oversee both the Lords and the Commons. Paul Martin, the first PSD, was ex MI5. There was another major upheaval in 2015 when parliament renegotiated its contract with the Met in a bid to save money. Initially there were rumours that a contractor such as G4S could be brought in. However, all the civilian personnel provided by the Met were then brought on to the parliamentary staff. The House insisted at the time that the number of officers was not being reduced. The most senior officer based on the site is thought to be an inspector. Advertisement

'These officers are the most vulnerable ... and the arrangements have got to be scrutinised to see if in future we need to have different arrangements surrounding them.'

He said there was already a project under way to improve security at the Carriage Gates.

Commons Leader David Lidington today said he was not aware of the 'war game' to test security.

He also denied that MPs had called for armed guards to be removed from Carriage Gates.

Mr Lidington told BBC Sunday Politics: 'The idea that there have been protests from MPs that led to any operational changes is simply not the case.

'What's happened in the last couple of years is that the security arrangements in New Palace Yard have actually been strengthened.'

He added: 'There are armed guards at all times in the secure estate of the Palace of Westminster, it is a matter for the security authorities and in particular for the police and direct command of those officers to decide how they are best deployed.'

Mr Lidington also denied there was a shortage of armed police in Parliament.

He said: 'I'm in no position to comment on the details operationally but my understanding is that the number of people available is what the police and the security authorities working together have decided to deploy, that they think was commensurate with the threat that we faced.'

Extensive interviews conducted with MPs and their staff in by Commons staff in 2014 revealed that many were unhappy with the level of security in place at the time.

A report prepared for the cross-party Administration Committee in April that year stated: 'Members also voiced concerns about the appearance of the security barriers surrounding the Palace.

'Some considered these to be extremely ugly and out of keeping with a major heritage site.

'It was noted that the stone barriers outside the Treasury and Foreign and Commonwealth Office were aesthetically pleasing and a similar style would be preferable outside the Palace of Westminster.

'Some Members and staff also noted that the presence of armed guards outside the main visitor entrances did not create a welcoming atmosphere.'

It added: 'The security barriers outside the Palace were regarded as ugly and the presence of armed police was off-putting.'

Lord Hain, who was Leader of the House of Commons between 2003 and 2005, said there had long been a cultural resistance to bolstering security at parliament.

He told MailOnline he was sat next to Tony Blair in 2004 when he was hit with flour thrown from the public gallery.

That scare sparked concerns that most of the Cabinet and Opposition frontbench could be wiped out by a single attack using ricin or anthrax.

But Lord Hain said he still had trouble persuading MPs that a glass screen should be installed in the public gallery.

'The situation has improved,' he said.

'It is clear that the Carriage Gates are the most vulnerable point.'