LONDON         -      Britain is threatening to deport European Union citizens if they fail to apply on time for a new immigration status after Brexit and will only grant leniency in exceptional circumstances, according to people briefed on the plans. In previously unreported guidelines, the government has drafted a narrow list of exemptions, such as those with a physical or mental incapacity, or children whose parents fail to apply on their behalf, the people said. The government is introducing the biggest shake-up of Britain’s border controls in decades, ending the priority given to EU migrants over those from other countries. Most EU citizens will need some form of prior permission from the government to remain in Britain. The government has often been accused of failing to establish who is living in Britain legally or illegally. In what became known last year as the Windrush scandal, thousands of Caribbean migrants were wrongly denied basic rights. Some lost jobs, others were wrongly deported. Government figures show just over half of the estimated 3.5 million European Union citizens living in Britain have received the new legal status ahead of the Dec. 31, 2020 deadline.