"Research suggests that vastly greater numbers of irregular migrants in the UK arrived by air rather than in the back of a truck", according to experts at Oxford University's Migration Observatory.
Those trying to cross by sea or road are well placed to do so from Calais, as it's both geographically close to Great Britain and a transport hub.
Generous welfare provision is an unlikely pull factor, since benefits aren't available either to undocumented migrants or asylum seekers, although the latter get a degree of state support and access to public services.
The effectiveness of our border security is hard to assess, but there have been past indications of poor performance.
"limitless welfare, free healthcare and education"
Migrants seeking to enter the UK without permission may be asylum seekers claiming to be at risk in their home country, or economic migrants with no claim to humanitarian protection. Asylum seekers are entitled to public services, plus accommodation and a basic living allowance if destitute, but only get the same benefits as UK citizens if protection is granted. Other immigrants who don't have permission to stay are entitled to very little.
Instead, asylum seekers can apply for government support if they are, or are likely to become, "destitute". This can be accommodation, a subsistence allowance, or both.
As the National Audit Office put it, the accommodation provided is "typically a flat or shared house in which the asylum seeker is provided with bedding and basic kitchen equipment as well as basic furniture and access to cooking and washing facilities", usually outside London and the South East.
Cash support to asylum seekers is limited to £36.95 per week for a single person aged 18 or over.
It's only after being recognised as a refugee that a person claiming humanitarian protection can claim benefits along the same lines as British citizens.
The Mail is correct that asylum seekers can use public services. As the House of Commons Library explains:
"Asylum seekers are eligible for free NHS healthcare and may be eligible for free prescriptions, free dental care, free eyesight tests and vouchers for glasses. Asylum seeker children have the same entitlement to state education as other children and may be eligible for free school meals".
There are, by the way, plenty of asylum seekers in Italy. The figures for the beginning of this year suggest that there were twice as many first-time applicants for asylum there as in the UK.
Illegal immigrants can't generally claim British benefits. As "persons subject to immigration control", they are specifically excluded from the likes of working tax credit, housing benefit and jobseeker's allowance.
This is difficult to measure, as for obvious reasons it's hard to get a fix on the number of illegal immigrants in the UK.
The National Audit Office did find in 2013 that the Border Force was consistently not meeting its target on the number of people refused entry at the border. In the same year, the Public Accounts Committee noted that checks on lorries at Calais were sometimes suspended to prioritise the checking of passports, although the Border Force has said it's since addressed this.
Research has suggested that the UK may have the largest population of illegal immigrants in the EU (between 417,000 and 863,000, including children), but this estimate is now very dated.
In any case, it doesn't necessarily tell us much about the quality of border controls. The Migration Observatory at the University of Oxford says that that "the consensus among researchers is that the majority of irregular migrants in the UK are likely to be visa over-stayers", rather than people who have snuck in.
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