At least five British people were gunned down on beach in Sousse, Tunisia

It follows three separate terror attacks in Tunisia, France and Kuwait today

Many of those killed when a gunman massacred holidaymakers on a Tunisian beach are British, the Prime Minister has confirmed.

David Cameron warned that the public needs to be prepared for the fact that 'many of those killed' in the 'savage' Tunisian shooting were Britons.

He spoke as it was confirmed that 15 of the 38 people people killed in the popular resort of Sousse were from this country.

David Cameron speaking live from Downing Street this morning. He described the Tunisia attacks as 'savage'

Cameron's speech came as it was revealed that a plan to detonate a bomb at today's Armed Forces Day march in Guilford (pictured) had bee foiled

Mr Cameron said a 'full deployment team' of consular staff, police and experts from the Red Cross would arrive in the North African country today to help the victims and their families and the Government was doing 'all we can to' help them.

Speaking in Downing Street, he said: 'These savage terrorist attacks in Tunisia, Kuwait and France are a brutal and tragic reminder of the threat faced around the world from these evil terrorists.'

The Prime Minister's update comes as police revealed a plan to detonate a homemade bomb at a military march in London today was foiled.

The Met have confirmed they were supplied with a dossier of evidence detailing ISIS' plans to kill and maim at an Armed Forces Day gathering near the barracks outside which solider Lee Rigby was murdered two years ago.

This morning's march, set to begin at 10am, was targeted by a former Birmingham-based jihadist, who told a man he believed to be an extremist how to construct a deadly DIY explosive almost identical to the one used in the Boston bombings in 2013.

It is alleged that a leading figure in ISIS, named as Junaid Hussain, told the undercover Brit: 'It will be big. We will hit the kuffar (unbelievers) hard InshAllah. Hit their soldiers in their own land. InshAllah. Soldiers that served in Iraq and Afganistan will be present. Jump in the crowd and detonate the bomb.'

A plot to target an Armed Forces march close to the barracks of Fusilier Lee Rigby has been foiled, police today confirmed

Police today confirmed that they had been made aware of the terror plot, which was planned for just 24 hours after 38 holidaymakers - the majority of whom were British - were gunned down by Islamic extremists as they sat in the sun on a beach in Sousse, Tunisia.

The South London plot was planned to be just as deadly, but instead of using guns the IS kingpin behind it wanted his suicide bomber to inflict widespread carnage by detonating a bomb put together using household items - including a pressure cooker, nails and rat poison.

A police spokesperson said: 'We are working closely with the event organisers of the Armed Forces Parade in Merton. There is a policing plan for the event and we will have officers on patrol as part of this plan.

'The police, together with our security partners, remain alert to terrorist threats that may manifest here or where individuals overseas may seek to direct or inspire others to commit attacks in and against the UK.'

The blast was intended to strike soldiers from the unit of murdered Lee Rigby, according to The Sun, who were set to participate in the march alongside 250 other serving soldiers and veterans.

The Met fears police officers may be targeted in a Lee Rigby-style attack, in which Michael Adebolajo (pictured) nearly decapitated the soldier in 2013

When the same event took place last year, more than 1,000 people attended, including members of the public.

However, the plot failed after the ISIS leader in Syria unwittingly recruited an undercover investigator from the newspaper to carry it out.

In communication made using the Kik mobile phone messaging app, Hussian said: 'They think they can kill Muslims in Iraq and Afghanistan then come back to the UK and be safe.

'We'll hit them hard InshAllah.'

The parade was targeted because it was closest to the barracks in Woolwich, south east London, where Fusilier Rigby, 25, was murdered by Islamist extremists in 2013.

Fusiliers from his regiment, serving Gurkhas and war veterans will be among today's marchers.

A Scotland Yard spokesman said: 'The police, together with our security partners, remain alert to terrorist threats that may manifest here or where individuals overseas may seek to direct or inspire others to commit attacks in and against the UK.'

Under threat: Security sources said Islamist fanatics are determined to carry out an atrocity against a police officer or another member of the Armed Forces (file photo)

Armed Forces Day is the annual commemoration of the service of men and women in the Army, Navy and RAF.

Britain's terror threat level remains at severe after yesterday's ISIS attacks in France, Kuwait and Tunisia.

Guidance has been issued to police that Islamist fanatics are determined to carry out an atrocity against either on duty officers or members of the Armed Forces.

To that accord, police have even been warned not to wear their uniforms in public when they are off duty, or to tell strangers they work for the police to protect them and their families from a jihadi attack.

The Met Police's Deputy Assistant Commissioner Helen Ball, said: 'Our priority is to keep people safe and we continue to review our ability to prevent and respond to terrorist incidents and potential threats.

'And as we approach a weekend of large public celebrations around the country, including Armed Forces Day events and Pride London, we will be putting in place additional security measures to help protect and reassure the public.'

Last night Foreign Secretary Philip Hammond chaired a meeting of police and security officials at the government's emergency committee, Cobra.

An official said the meeting considered the 'impact overseas and the impact domestically' of the spate of atrocities.

Terrorist: Images show the final moments of a gunman (pictured) who caused havoc by opening fire on a beach packed with holidaymakers in Tunisia today, before police shot him dead

Massacre: At least 37 people were killed on the beach in Tunisia when gunmen opened fire on innocent sunbathers

Sickening: Meanwhile a victim's head was found hanging on a fence near a factory in France, flanked by two Islamist flags

Location: The attack took place in Saint-Quentin-Fallavier, close to the city of Lyon in the south east of France

Carnage: Medics and security forces gather inside the Imam Sadiq Mosque attempt to secure the scene following the devastating blast in Kuwait

It is feared the attacks are linked to a message broadcast by Islamic State in calling for Muslims to make the month of Ramadan one of 'calamity for the infidels'.

The message, broadcast by IS representative Abu Muhammad al-Adnani, called for 'calamity for the infidels … Shi'ites and apostate Muslims'. Whitehall officials warned the threat level in Britain was likely to remain at severe – meaning an attack is 'highly likely'.

David Cameron said the attacks could have happened 'anywhere' in the EU, as he condemned the terrorists' 'twisted and perverted ideology'.

Speaking in Brussels, where he talked briefly to French president Francois Hollande about the 'appalling' attack in Grenoble, the Prime Minister said: 'We have to combat not only the terrorism, not only working with the countries that are suffering, but dealing with this poisonous mindset, this death cult that is poisoning young minds and turning them to this path of mindless violence.

'People who do these things sometimes say they do it in the name of Islam. They don't. Islam is a religion of peace.

'They do it in the name of a twisted and perverted ideology that we have to confront with everything that we have.'

Fear: The terror attacks in Kuwait, France and Tunisia may have been part of ISIS's plans to mark the holy month of Ramadan with an unprecedented spate of violence 'against the non-believers'

But Mr Cameron suggested Europe remained too complacent about the deadly threat posed by Islamist extremists. British officials said he was frustrated that longstanding plans to force airlines to collect and share data on passengers flying into the EU are still 'stuck in the European Parliament'.

Meanwhile, two security advice booklets were distributed to all forces yesterday morning by the College of Policing.

They advised officers and staff to cover up their uniforms when travelling to work and when leaving stations or offices.

The guidance – long in planning – was drawn up by Centre for the Protection of National Infrastructure. The timing of its launch was not directly linked to yesterday's attacks.

The advice to police follows advice that British soldiers should not wear their uniform outside barracks. They have also been told not to let strangers know that they are in the Army.

That instruction was issued after Whitehall officials noticed increased 'chatter' among jihadists wanting to carry out a beheading or other high-profile attack on a serving soldier.