Police are reviewing the security of all defected Russian spies living in the UK in the wake of the attack on Sergei Skripal, officials have revealed.

Sir Mark Sedwill, the national security adviser, said investigators have not yet identified who poisoned the former double agent and his daughter with nerve agent in Salisbury.

“The police who are responsible for protective security and the various agencies alongside them are reviewing the security of all people who might be vulnerable,” he told the Defence Committee.

Asked whether those responsible for the attack on Mr Skripal and his daughter Yulia had been identified, he replied: “Not yet, no.”

But Sir Mark insisted that authorities had acted “much faster” than following the murder of another former Russian spy, Alexander Litvinenko, with radioactive polonium in 2006.

Mr Skripal, a 66-year-old former military intelligence colonel, had been living openly under his real name after being handed over in a 2010 spy swap and the attempted assassination increased fears held by other defectors.

Little over a week after the attack in Salisbury on 4 March, Russian businessman Nikolai Glushkov was found strangled at his London home.

Nikolai Glushkov, a businessman convicted of fraud in Russia, was murdered in his London home (PA/Metropolitan Police)

No one has been arrested in relation to his death and police investigations continue.

Scotland Yard said there was no immediate evidence of a link to the attack on Mr Skripal, but Mr Glushkov’s links to Russian dissident Boris Berezovsky sparked fresh scrutiny over a string of suspicious deaths.

Sir Mark told MPs that Russia posed a wider “existential threat” to the UK, both through its nuclear capability and unconventional warfare.

“What we’ve seen is more aggressive Russian behaviour, the development of hybrid warfare, and upgrades to their conventional military capabilities – some of them designed to threaten our own deterrent that quite rightly means we’re shifting the focus of defence,” he said.

“As we’ve seen most acutely in Salisbury, but with a range of other activities both in the UK and elsewhere, there is very aggressive cyber activity, subversion, money flowing into politics, information operations etc, the Russians are operating aggressively just below the level of armed conflict and that affects our national security.”

He was grilled for more than two and a half hours on Britain’s defence capability.

Several MPs have called for the UK to increase security funding and modify plans originally drawn up several years ago in light of the changing threats posed by cyber attacks, hybrid warfare and terrorist groups such as Isis.

Sir Mark told the Defence Committee spending was already increasing year on year and was above Nato’s benchmark of 2 per cent of GDP, saying allies should increase their own contribution.

Skripal attack aftermath – in pictures Show all 15 1 /15 Skripal attack aftermath – in pictures Skripal attack aftermath – in pictures Scene of attack Members of the emergency services in hazard suits fix the tent over the bench where Sergei and Yulia Skripal were found unconscious on a park bench in Salisbury in March 2018. Getty Skripal attack aftermath – in pictures Victim - Sergei Skripal The retired Russian colonel and former double agent for MI6 was in a critical condition in hospital for more than two months after being exposed to novichok in Salisbury. He was given refuge in the UK after being jailed in Moscow for treason. Mr Skripal came to Britain as part of a high-profile “spy swap” in 2010 in which four men were exchanged for ten Russian "sleeper agents" in the US. In this image he is speaking to his lawyer from behind bars in Moscow in 2006. AP Skripal attack aftermath – in pictures Victim - Yulia Skripal Yulia Skripal was struck down by a novichok poison alongside her father Sergei. Facebook Skripal attack aftermath – in pictures Scene of attack A police officer stands guard outside a branch of the Italian chain restaurant Zizzi where the pair dined at before falling ill. It was boarded off whilst investigators worked on the building and later found traces of the chemical weapon within it. AFP/Getty Skripal attack aftermath – in pictures Scene of attack Large areas of central Salisbury were cordoned off by police following the discovery of the Skripals. Traces of nerve agent were also found in The Mill pub. PA Skripal attack aftermath – in pictures Victim - Nick Bailey Detective Sergeant Nick Bailey, rushed to the aid of the Russian ex-spy and his daughter who were targeted with a nerve agent. He was hospitalized after aiding them and didn't leave until three weeks after the attack. Wiltshire Police/Rex Skripal attack aftermath – in pictures Police investigation - Skripal’s home Police believe they were poisoned at home, and detectives found the highest concentration of novichok on the front door of Mr Skripal’s house. Getty Skripal attack aftermath – in pictures Theresa May visits scene of attack Britain's Prime Minister Theresa May spokes with Wiltshire Police's Chief Constable Kier Pritchard near where the Skripal's were found. Britain expelled 23 Russian diplomats over the nerve agent poisoning and suspended high-level contacts, including for the World Cup on March 14. Theresa May told parliament that Russia had failed to respond to her demand for an explanation on how a Soviet-designed chemical, Novichok, was used in Salisbury. AFP/Getty Skripal attack aftermath – in pictures Skripal days before attack Sergei Skripal days before he was exposed to Novichok, that has left him fighting for life. ITV News Skripal attack aftermath – in pictures Police investigation - military involvement British soldiers were deployed soon after the attack to help a counter-terrorism investigation into the nerve agent attack. One of the places they were asked to help out with was Skripal's home and it's surrounding. They were asked to remove a vehicle connected to the agent attack in Salisbury, from a residential street in Gillingham. AFP/Getty Skripal attack aftermath – in pictures Police investigation Personnel in protective coveralls and breathing equipment cover an ambulance with a tarpaulin at the Salisbury District Hospital. AFP/Getty Skripal attack aftermath – in pictures Police investigation The investigation extended to the grave of Sergei Skripal's son Alexander in London Road cemetery. Getty Skripal attack aftermath – in pictures Police investigation The Counter Terrorism Policing Network requested assistance from the military to remove a number of vehicles and objects from Salisbury. EPA Skripal attack aftermath – in pictures Home Secretary visits scene of attack Home Secretary Amber Rudd visited the scene of the nerve agent attack at the Maltings shopping centre on 9 March. Getty Skripal attack aftermath – in pictures Yulia Skripal speaks for the first time Yulia Skripal, speaking for the first time, said she felt lucky to have survived the nerve agent attack in Salisbury which left her fighting for life. Ms Skripal said her life had been “turned upside down” by the assassination attempt. But the Russian national added she hoped to return to her homeland one day, despite the Kremlin being blamed for the attack. Reuters

“Nato needs to spend more on defence – it’s not just the UK up against Russia, it’s Nato that’s the lynchpin of our defence,” he added.

“I’m not arguing for or against increase in defence budget.”

There has been heavy criticism over delays to the F-35 aircraft programme and new aircraft carriers, with Sir Mark admitting the ships will have to be escorted in contested waters by other countries’ assets.

He insisted the UK was still a “major player” in terms of global military capability, despite “eye-watering” gaps between what the Ministry of Defence wants to buy and can afford.

Sir Mark admitted that there were areas he would personally like to invest in but refused to name them or be drawn on whether he had discussed extra cash with Theresa May.

“There are areas of vulnerability across the entire national security architecture, not just in government, a lot of this is outside government as well,” he said.

“Given the nature of modern warfare and the nature of non-state threats... yes I would like to invest.”

The grilling came after Gavin Williamson, the defence secretary, called on people from technology and communications professions to join the UK’s reserve forces to help combat disinformation campaigns.

In an interview with The House magazine, he said that army recruitment should be about “looking to different people who maybe think, as a journalist: ‘What are my skills in terms of how are they relevant to the armed forces?’