Britain will deport EU citizens that fail to apply on time for a new immigration status, and leniency will be reserved for exceptional circumstances, Reuters reports, citing unpublished policy guidelines.

The government’s unpublished plan envisages the most significant overhaul in the UK’s immigration regime in decades, ending the priority status of EU citizens. Exemptions to cases of physical or mental incapacity, or children whose parents fail to apply on their behalf. The plan could be revised if the Conservatives lose the December 12 elections.

At the moment, just over 50% of the 3,5 million citizens living in the UK have applied for a new legal status ahead of the Dec. 31, 2020 deadline. Should the registration program, tens of thousands could face deportation.

A study published in March suggests that, historically, new registration systems for large populations have mixed success: less than 20% uptake in the UK, 43% in the United States, and 77% in Spain. The only scheme that came near to a 100% take-up was an ID card system in India.

Campaigners for EU citizens rights in the UK fear that those left behind are usually the most vulnerable, including the elderly, those not in regular employment and victims of domestic violence. Even a modest 5% registration failure would be equivalent to 175,000 people.