LONDON (AP) — Britain gained 333,000 people through immigration in 2015 — a near-historic high that is fanning the debate about newcomers ahead of a vote next month on whether the country should remain in the European Union.

Those who back leaving the EU say Thursday's Office for National Statistics figures prove that Britain can't control immigration unless it quits the 28-nation bloc. EU citizens can live and work in the other member states — and Britain's relatively strong labor market has proven to be an irresistible draw for hundreds of thousands of workers from the continent.

"You see the pressure on public services, you see the waiting lists in hospitals, in GP surgeries and of course in schools," said former London Mayor Boris Johnson, the leading member of the "leave" camp. "People are feeling it and what they resent is the lack of control."

London Mayor Sadiq Khan aggressively argued for the "remain" side as the figures were released, arguing that the capital's strength lies in being open to the world.

"We wouldn't be where we are today in London with an isolationist approach," he said. "We know from our experience that the answer is to get more involved, to form more alliances and actively to shape our future in the world. That is the British way. That is certainly the London way."

Prime Minister David Cameron, who backs the "remain" side, has long promised to reduce net migration below 100,000. His failure to do so is giving ammunition to EU "leave" activists.

The statistics office says the U.K. had 630,000 immigrants in 2015, while 297,000 people left. The net figure of 333,000 is 20,000 higher than in 2014 — a statistically insignificant change — and the second-highest on record.

With less than a month to go before the June 23 vote, the immigration figures also give the "leave" camp ample opportunity to change the subject. The economy has dominated the discussion so far, with economists at the International Monetary Fund, the Bank of England, and several respected think tanks warning that leaving the EU's single market of some 500 million could hit British businesses and consumers hard and may lead to a recession.

The "leave" camp has already tried to turn the debate toward the more populist theme of sovereignty. Johnson at one point compared the EU's aims to those of Adolf Hitler — arguing that the bloc wants to create a superstate that mirrors the Nazi leader's attempt to dominate the European continent.

That was too much for the president of the European Commission, Jean-Claude Juncker, who on Thursday suggested that Johnson had gone too far in his claims that Britain would be better off outside the EU. Speaking at a G-7 summit in Japan, Juncker noted that Johnson had spent time as a Brussels-based journalist and said that he should return to check "if everything he is telling the British people is in line with reality — I do not think so."

The British government, meanwhile, urged people to be realistic. James Brokenshire, the immigration minister, stressed that there are no easy solutions, regardless of whether Britain stays or goes.

"Leaving the EU is absolutely no panacea or silver bullet, whatever some may suggest," he said.