Reality Check: Could a million EU workers get residency rights?
The claim: Allowing EU migrants who arrive during the transition period the right to stay would mean a million people have that right.
Reality Check verdict: Potentially, a million people could come, but it's unlikely to be that high. Migration Watch's estimate is based on previous numbers of EU workers applying for National Insurance numbers in the UK, which included many people who travelled to the UK for short-term work and have now left the country. On the other hand, they exclude non-working dependants.
Alp Mehmet, from Migration Watch UK, which campaigns for controls on migration, told BBC Radio 4's Today programme on Thursday morning that allowing people arriving from the EU during the transition period the right to stay indefinitely and bring their dependents "potentially means a million people" having such rights.
He based that on figures for the numbers of EU nationals applying for National Insurance numbers, which he said had been roughly half a million a year in recent years (and transition is expected to last about two years).
The government and the EU are negotiating what happens during the transition period.
And Prime Minister Theresa May has indicated she will fight a proposal to give residency rights to EU citizens during the period after Brexit.
What are the figures?
Everybody needs a National Insurance number (Nino) to work in the UK.
The most recent figure from the Department for Work and Pensions was for NI numbers allocated in the year to the end of September 2017, when the number was 548,000, and it had indeed been above half a million for at least the three years before then.
But those figures were considerably higher than the migration statistics from the Office for National Statistics, the most recent year of which showed 230,000 people arriving from the EU and a net figure (after subtracting people leaving) of 107,000.
The migration statistics count people arriving in the country who plan to stay for at least a year, and those who have been in the country for at least a year and leave.
There was much discussion in 2016 of why there were so many more EU nationals getting National Insurance numbers than had been arriving in the UK.
The ONS looked into it and said the difference was due to short-term migrants.
The migration figures cover people planning to stay for at least a year, whereas people applying for a National Insurance number could stay for shorter periods.
One of the people questioning the size of the difference between the two figures was Prof Jonathan Portes, from King's College, London, who thinks more work is needed into why the discrepancy exists, especially because the migration figures have been falling faster than the National Insurance numbers.
On the other hand, the National Insurance number figures cover only migrants wanting to work - their non-working dependants would be included in the ONS migration figures but not in the National Insurance figures.
The key problem with the National Insurance figures is they include people who have worked for a short time and then left - in order to qualify for "settled" status under the current plans they would need to be in the country for five years.
The proposal for settled status, outlined in the joint negotiated report between the EU and the UK, is that EU nationals who have been in the UK for five years (starting before the UK leaves the EU) will be eligible for the status, giving them indefinite leave to remain with the same access to public services as they have now.
BBC Radio 4's Today programme presenter Nick Robinson, who was interviewing Mr Mehmet, countered the Nino figures with a figure of net migration from EU countries of about 9,000 a quarter.
The ONS generally publishes the migration statistics as annual figures, but this quarterly one came from an ad hoc request and is for April to June 2017.
The figure that is normally quoted is that for the year to the end of June, which was 107,000.