The first applications from EU nationals wanting to stay in the UK after Brexit are being submitted.
A group of university students and NHS workers in the north-west of England are taking part in a trial to test the system before it opens later this year.
The three million or so EU residents in the UK have until the end of June 2021 to register for "settled status".
Ministers, who insist the process will be simple, say the pilot will allow for "necessary adjustments" to be made.
The UK is due to leave the EU on 29 March 2019.
The government has said it wants EU nationals living in the country to be able to stay, with Brexit Secretary Dominic Raab saying last week that this applied whether there was an agreement on the terms of exit or not.
Ministers have said they aim to keep paperwork to a minimum, saying the form that has to be filled in online or via smartphone app will contain only a handful of questions and take a matter of minutes to complete.
Up to 4,000 people, including students from Liverpool universities and staff from 12 NHS trusts, will be the first to have a go during the pilot. They are being guided through the application process by Home Office officials.
Applicants have to provide proof of identity, declare any criminal convictions and upload a photograph - while officials will check employment data and run security checks.
The process - which will cost £65 for adults and £32.50 for children under the age of 16 - will operate on the basis of a presumption that applications will be accepted.
While stopping short of guaranteeing their future, Mr Raab said last week that it was "inconceivable" that non-UK citizens would be asked to leave whatever the outcome of current negotiations with the EU.
Immigration minister Caroline Nokes said the pilot was another step in making it easy for EU nationals to obtain settled status - enabling them to live and work in the UK on the same terms as now.
"From today, we are inviting a small group of EU citizens to make an application to secure their status," she said. "We will use their feedback to make any necessary adjustments ahead of the scheme being fully opened."
The scheme is due to fully open on 30 March, 2019, with settled status granted to EU nationals who have lived in the UK for five years - unless they have serious criminal convictions or for security reasons.
Those who are resident in the UK by 31 December 2020 but who have not lived in the UK for five years will get pre-settled status which allows them to live and work in the UK until they reach the five year mark and can claim settled status.