David Cameron has suggested Islamic State (IS) would be happy if the UK votes to leave the European Union on June 23.
Answering questions after delivering a myth-busting speech on the EU referendum at Mansion House in the City of London, the Prime Minister posed a rhetorical question asking "who would be happy" if the nation does vote for Brexit.
He then said Russian president Vladimir Putin would welcome such an outcome, before adding that "I suspect al-Baghdadi" would, too.
Abu Bakr al-Baghdadi is the leader of the IS terror group, also known as Isis and Daesh.
Mr Cameron said: "It is worth asking the question: Who would be happy if we left?
"Putin might be happy, I suspect al-Baghdadi might be happy.
"Our friends around the world are giving us a very clear message, they are saying 'it's all up to you, it is your sovereign choice'.
"But our friends in Australia and New Zealand and America and all around the world and all round Europe, they are saying 'it's all up to you, its all your choice, but we would like you to stay, we think it's good for us and it's good for you'."
Boris Johnson, during a visit to Stafford, told reporters he believed it was "a bit much" to suggest IS is an ally of Leave supporters.
The prominent Vote Leave campaigner said he did not believe in getting involved in "artificial media twit storms", adding: "One might argue it's a bit much to start comparing people arguing for freedom in this country or the restoration of democracy in this country to say our allies are Putin and Daesh.
"I think that's a bit much, really."
Both sides of the referendum campaign have been criticised by the other for apparent scaremongering.
But today marks the first time that the Prime Minister has explicitly cited Islamic State as a reason for voting to Remain.
Mr Cameron also faced questions following his speech regarding the EU's deal with Turkey which has been criticised by former spy chief Sir Richard Dearlove.
But the Prime Minister rejected the idea that the deal is dangerous.
He said: "No it isn't. What the deal did was that there were tens of thousands, hundreds of thousands of people leaving Turkey, paying people smugglers, getting into boats, arriving in Greece and therefore being given the right to settle as asylum seekers or refugees in Europe.
"I said repeatedly to my colleagues in Europe, 'look, we're not in your no borders zone, we are going to maintain our border, if you let these people in that doesn't mean they are going to be able to come to Britain so in some ways this is not my business but my very strong advice is that you need to have a policy where people who arrive in Europe can immediately be sent back to Turkey'.
"Why? Because you have got to break the model of the people smugglers.
"As long as they can make that trip the people smugglers go on having a business.
"Once you send them back and say 'Turkey is a safe country, we can examine your claim for asylum or refuge there in Turkey', once you do that you break the model of the people smugglers."
Mr Cameron said that the deal with Turkey represents the "best of both worlds".
"We are not in the no border zone so these people don't have a right to come to Britain," he said.
"We are taking our refugees directly from the camps rather than from other European countries.
"But we had an influence on trying to help Europe towards getting to the right decision."
Sir Richard had suggested that giving visa-free access to the EU to millions of people in Turkey would be like "storing gasoline next to the fire" in relation to the migration crisis.
Addressing Sir Richard's comments directly, Mr Cameron said that during most of his time as Prime Minister, John Sawers was the head of MI6 and Jonathan Evans was the head of MI5, and those individuals had helped stop "bomb plot after bomb plot, attack after attack, conspiracy after conspiracy".
He said: "I have huge respect for them and the work that they did to keep this country safe.
"Those two individuals are now saying as clearly as they possibly could that Britain is safer from terrorism if we stay in the European Union."
Mr Cameron said he is not suggesting membership of the EU is the only thing keeping the UK safe, "but it helps".