Theresa May has refused to condemn Iain Duncan Smith's suggestion that the UK's membership of the EU leaves the country vulnerable to a Paris-style terrorism attack.

Andy Burnham, the shadow home secretary, accused the Work and Pensions Secretary of "baseless scaremongering" as he asked his counterpart to reject the idea put forward by her cabinet colleague.

Mr Burnham also took a thinly veiled shot at Boris Johnson for announcing his intention to campaign for Brexit as he poked fun at Mrs May's own announcement that she will campaign for the UK to remain in the EU.

He said: "Can I commend the Home Secretary on her announcement this weekend and on her decision to put the national interest before self-interest unlike others.

"When you began as Home Secretary you took a Eurosceptic stance, opting out of dozens of EU measures.

"But you have since opted back in to many, most recently on the sharing of fingerprinting and DNA.

"Is it fair to say that the realities of office have shown you the value of EU co-operation in tackling crime and terrorism and changed your mind on Britain's membership of the EU?"

Mr May replied: "I have always been very clear about the value of co-operation when it is in the British national interest.

"That is why of the 35 measures that we decided to propose to this House we should opt back in to in relation to protocol 36 on justice and home affairs measures, it was precisely because we believed those were in the national interest."

Mr Burnham then said he would take the Home Secretary's answer as a "yes" to his initial question before launching an attack on Mr Duncan Smith who said at the weekend that the UK's "open border" meant there was a lack of control over people entering the country.

Mr Burnham said: "Yesterday on the Marr programme the Prime Minister was explicitly clear that our membership of the EU helps Britain fight terrorism.

"But within minutes he was directly contradicted by one of his own cabinet ministers who claimed the UK's EU membership made a Paris-style attack here more likely.

"This would be bad coming from Ukip but coming from one of the most senior members of the cabinet it is downright irresponsible.

"Will the Home Secretary take this opportunity to reaffirm Government policy on this crucial issue and condemn this baseless scaremongering?"

Mrs May said the Government's "position on this issue is very clear".

She said: "I'm very clear that there are many areas in which that co-operation with other member states in the European Union is of our benefit in terms of the national security of this country in dealing with criminal matters.

"As I indicated earlier in response to questions we do of course take security at our border very seriously and that's why we have the checks that we do at our border."