LONDON — This week Prime Minister Theresa May received the backing of Britain's European allies after the poisoning of Sergei Skripal and his daughter Yulia in Salisbury.

European Council President, Donald Tusk, was one of the first to offer his "full solidarity" to the UK and said he would make the attack a talking point at next week's Council summit in Brussels.

As a result campaigners against Brexit have started to raise questions about whether Britain will be as capable of combatting Russian aggression after it leaves the European Union.

Research conducted by the campaign group Open Britain and given to Business Insider highlights how May's government's most senior ministers have previously argued that Britain is safer from hostile states like Vladimir Putin's Russia while inside the EU.

Boris Johnson

Matt Cardy / Stringer , foreign secretary and Leave campaigner Boris Johnson warned that leaving the bloc would empower Putin to be more aggressive on the world stage.

"There is the Putin factor: we don’t want to do anything to encourage more shirtless swaggering from the Russian leader, not in the Middle East, not anywhere."

David Lidington

Britain's Minister for Europe Lidington speaks during the "Rome 2015 MED, Mediterranean dialogues" forum in Rome Thomson Reuters , Minister for the Cabinet Office, David Lidington, emphasised the impact EU sanctions have had on Russia.

"The UK has played a leading role in pressing for tough EU sanctions, delivering a cost to Russia for its actions and supporting the full implementation of the Minsk agreements."

Four years earlier, he said that the EU had "unique selling points" for dealing with foreign aggressors which other international bodies, including NATO, do not possess.

"But the EU has its own unique selling points when it comes to international security. Its advantage lies in the broad and complementary tools it can use - diplomatic, civilian, military, developmental, and financial - which it can apply collectively to promote international peace, often in places where NATO and others cannot act."

Amber Rudd

Britain's Home Secretary Amber Rudd leaves 10 Downing Street, London, December 5, 2017. REUTERS/Hannah McKay Hannah McKay/Reuters , Home Secretary Amber Rudd suggested that the EU possessed a level of power to combat Russia that the UK alone did not.

"As a bloc of 500 million people, we have the power to force Putin’s hand."

Philip Hammond

Conservative party leader Theresa May and Chancellor Philip Hammond take questions during a general election campaign event in East London. Stefan Rousseau/PA Wire/PA Images The Chancellor has been clear in his view that EU-wide sanctions are the best means Britain has at its disposal for protecting itself from aggressive moves from the Kremlin. A week before the Brexit referendum, he said:

"And when we face challenges to our security like Russia’s annexation of Crimea and de-stabilisation of Eastern Ukraine; or like Iran’s attempt to build a nuclear bomb, the most effective weapon we have found to counter them has been EU-wide sanctions."

and finally... Theresa May

Like many of her ministers, the prime minister has emphasised the importance of EU membership when it comes to national security. In January 2016, May said:

"The Government are clear that we must protect the UK and her interests from Russia-based threats, working closely with our allies in the EU and NATO."

In a speech during the referendum campaign she also suggested Brexit would leave Britain more vulnerable to international crime and terrorism.

"My judgment, as Home Secretary, is that remaining a member of the European Union means we will be more secure from crime and terrorism," she said.