A former CIA chief has warned that European Union membership sometimes "gets in the way" of states providing security, fuelling the row over whether a Brexit would put the safety of Britons at risk.
Retired general Michael Hayden said the EU was "not a natural contributor to national security" in member states as he backed former MI6 chief Sir Richard Dearlove's claim that the cost of Brexit would be low and there could be benefits.
The former US spy chief said Brexit would have little impact on America's willingness to work with the UK or other European intelligence agencies.
Home Secretary Theresa May and Defence Secretary Michael Fallon have both insisted that EU membership contributes to the UK's security, and Sir Richard's comments were contradicted by other intelligence experts including former GCHQ chief Sir David Omand.
But Gen Hayden said "Sir Richard is right", adding :"The union is not a natural contributor to national security of each of the entity states and in some ways gets in the way of the state providing security for its own citizens."
He told BBC Radio 4's Today programme the capabilities of the EU states' intelligence agencies were "very uneven", with the much-criticised Belgian security services described as "small, under-resourced" and "legally limited".
He said: "National security remains a national responsibility and so, sadly, the grades you have to give to each for the services are individual and it's very uneven.
"Very good services, aggressive services in France, Great Britain; good but small services in Scandinavia and then, unfortunately, throughout most of the rest of the continent small services and, if you are talking specifically about Belgium, small, under-resourced, legally limited and frankly, working for a government that itself has its own challenges in terms of overall governance."
The European agencies co-operated more with the United States than their counterparts on the continent, he said.
"Frankly, we are actually a pretty good investment. We are a huge security service and each sees their national interest as being well served by having a productive relationship with us and frankly the same math does not apply to other services on the continent."
Gen Hayden's comments were seized on by Brexit campaigners.
Leave.EU co-founder Arron Banks said: "Now that a former head of the CIA, a security superpower, has shot down claims that the EU makes us safer, Cameron has no excuse for not being straight with the British public.
"General Hayden, Sir Richard Dearlove and the man in the street all understand that we're made less safe by EU rules which give Belgian Islamists the right to settle in the UK and EU courts which stop us from deporting dangerous people who are already here."
One of the other main issues in the debate ahead of the June 23 referendum on EU membership has been migration, and new analysis suggested that EU citizens in the UK were more likely to have jobs than their British counterparts.
The study found 83% of migrants from the newest EU members - including the eastern European states - were in work, as were 75% from the other 14 EU countries, compared with 74% for UK nationals and 62% for non-EU migrants.
But the analysis showed central and eastern European migrants tended to be in low-skilled, poorly-paid jobs, earning an average of £3 an hour less than UK nationals.
The Institute for Public Policy Research (IPPR) paper found EU migrants are less likely to claim unemployment or sickness benefits than UK nationals, but more likely to report claiming tax credits and child benefits.
IPPR research fellow Marley Morris said: "Our new analysis of the data on employment, welfare and housing paints a mixed picture of the impacts of EU migration on the UK.
"A large majority of EU migrants are in work and so are paying taxes rather than living off out-of-work benefits, but they are also more likely to be claiming in-work benefits than others in the workforce.
"Many eastern Europeans, despite their qualifications, are working in low-skilled sectors at low pay rates, which may be helping to plug some labour shortages but might also be sustaining low wages and poor conditions in some workplaces.
"Our analysis also suggests that EU migrants are more likely than others to live in the private rented sector, but that doesn't mean they aren't able to access scarce social housing. In fact their likelihood of living in social housing is about the same as the general population."