Theresa May has vowed to make it as easy as possible for EU citizens to remain in the UK after Brexit ahead of a key summit of European leaders.
In a Facebook post, the prime minister insisted the application process for settled status would be "streamlined" and the cost "as low as possible".
She said representatives of EU citizens will sit on a "user group" which will iron out any problems in the system.
The other 27 EU leaders will assess overall progress in the talks so far.
At a meeting on Friday, at which the UK will not be present, they are expected to conclude officially that "insufficient progress" has been made on the status of EU nationals in the UK and British expats on the continent - and other separation issues - to move onto the second phase of trade discussions.
European Council President Donald Tusk said there would be no "breakthrough" at the two-day summit, but progress could be achieved by the next scheduled meeting of EU leaders in December.
Mrs May, who will address other leaders at a working dinner on Thursday, wants mutual dialogue on the UK's future relationship with the EU, including trade and defence, to begin as soon as possible.
In the event of no progress at Thursday's meeting, the letter says, Mrs May should formally declare the UK is working on the assumption it will be reverting to World Trade Organisation (WTO) rules on 30 March 2019.
Early notification of such a move would allow the UK to "concentrate our resources on resolving administrative issues" and prepare to "crystallise the economic opportunities" of Brexit, it adds.
Meanwhile, a trade body for the UK's creative sector has warned that money generated by it could be hit by a post-Brexit restriction on immigration.
The Creative Industries Federation (CIF) said the £87bn a year that UK-made films, music, adverts and video games generated for the UK economy was at risk if immigration was restricted.
The sector relies heavily on freelance staff, many of whom are from the EU, and the CIF wants the government to negotiate free movement of UK and EU workers for short-term projects.