Millions of EU nationals should not have to pay to register to stay in the UK after Brexit, the European Parliament's Brexit co-ordinator has told the Government.
Guy Verhofstadt has written to Home Secretary Sajid Javid, saying the £65 cost of signing up to the settlement scheme should be waived.
The fee for the EU citizens living and working in the UK is among a number of "outstanding concerns" outlined in the letter on behalf of the EP's Brexit Steering Group.
It is also concerned about possible barriers to "vulnerable" people signing up, and wants more information about the powers of the Independent Monitoring Authority (IMA) that will oversee the appeals process.
Mr Verhofstadt, the former Belgian Prime Minister, said: "MEPs remain unhappy that EU27 citizens in the UK, who have contributed to British society and paid their taxes, will have to pay for registration.
"We continue to believe this process should be cost-free, in keeping with the European Parliament's longstanding position on this matter.
"More needs to be done to ensure that vulnerable EU citizens are properly catered for and that any delays faced by citizens with the registration process itself does not create unnecessary anxiety."
The steering group is also to investigate the preparations made by EU nations for British citizens living there following Brexit, he added.
The £65 adult cost of the registration scheme for EU citizens wishing to stay in Britain after the end of the transition period on December 31, 2020 is the same as the current fee for a permanent resident document.
It is also £10.50 less than the minimum cost of a standard British passport.
It will cost £32.50 for children under 16 and be free for those that already have permanent residence documents.
After a small test run the scheme will be rolled out from late this year, with the aim of being fully open by March 30, the day after Britain leaves the EU.
A final deadline for applications has been set for the end of June 2021.
In the letter to Mr Javid, which has also been sent to Brexit Secretary David Davis and Yvette Cooper, the chairwoman of the Commons' Home Affairs Committee, Mr Verhofstadt said there were "a number of issues of concern".
He added: "We would urge that you take these on board during the consultation and testing phase of the scheme and before its official launch.
"Addressing them would also instil confidence on our side that the registration scheme will work in the streamlined and user-friendly way that we all want."