ON MARCH 7th European leaders announced that they had agreed with Turkey the outlines of the latest plan to ease the continent’s migration crisis. On the same day, Britain said its Royal Navy would join NATO efforts to intercept and return people attempting to reach Greece illegally by crossing the Aegean sea. Yet figures obtained by The Economist under the Freedom of Information Act (after an unexplained six-month delay) provide a reminder of problems closer to home.

According to the Home Office, the number of detected clandestine entry attempts to Britain via European ports and train tunnels increased from around 1,000 per month in 2008-12 to 2,000 in the late 2013, before rising to 4,000 in 2014 and almost 13,000 in July 2015 (see chart). The increase is caused both by a rise in the number of people trying to enter Britain—many of them fleeing war-torn parts of the Middle East and Africa—and by the fact that they are more tenacious. Most are now willing to try 11 or 12 times, up from five or six times a year ago, reckons Franck Duvell of the University of Oxford’s Centre on Migration, Policy and Society. The border is among the tightest in Europe since the Iron Curtain, he says. Lorries are X-rayed for stowaways, carbon-dioxide sensors detect human activity and sniffer dogs patrol regularly. The few who do manage to sneak in from Europe represent only a small fraction of the half-a-million or so people thought to be living illegally in Britain. (Most enter using false passports and papers or outstay their visas.) Indeed, those seeking entry via the Channel Tunnel and seaports are among the most likely to have a good case for asylum, since many are from countries that have high rates of successful applications. Common nationalities in a notorious refugee camp in Calais include Syrians, Eritreans, Sudanese and Iraqis. Many have family members already in Britain.

That will provide little solace to “In” campaigners in the EU referendum on June 23rd, who are aware that immigration from Europe is one of the main factors persuading people to vote to leave. In August 2015 there were 3,000 people in the Calais camp; there are now 4,000, with another 2,500 in Dunkirk. French authorities are currently dismantling the camps. But residents are unlikely to move on, says Zoe Gardner of Asylum Aid, a charity. The number of clandestine entry attempts may be even higher by the time voters head to the polls.