This startling front-page claim was made by the Brexit-supporting Sunday Express in May. If true, it would represent an unprecedentedly large wave of migration: At present, there are around 3 million EU citizens living and working in the UK, so a wave of 12 million more would be huge.
However, there is almost no basis at all to the story. Firstly (see above), Turkey is not likely to join the EU at any point in the near future, so the issue is moot.
The basis for the 12 million figure is also questionable: It comes from a survey in the country where people were asked "would you, or any members of your family, consider moving to the UK?" if Turkey became an EU member. Sixteen per cent of respondents said yes – leading to the 12 million figure.
But for someone to say they would consider moving is very different to saying that they or any one member of their extended family would think about moving. It's a bit like asking a roomful of people if anyone knows a teacher, then saying anyone who raises their hand is a teacher. It's not true.
The Sunday Express has subsequently "clarified" the front-page claim on page 2 of its most recent edition.