Theresa May's suggestion the Brexit transition period could extend beyond the end of 2020 is "problematic", an ex-Conservative minister has warned.
The plan would extend the time between leaving the EU next March and the start of new trading arrangements.
Mrs May said that period could be a "matter of months" longer to ensure no visible border in Northern Ireland.
Aberconwy MP Guto Bebb said the whole Brexit process was causing "real damage" to the government's competence.
The transition phase is designed to allow businesses to prepare for new trading arrangements after Britain leaves the EU on 29 March.
The prime minister said the suggestion of extending the transition period was a new idea that had emerged in negotiations and was not expected to be used.
Some Conservatives MPs are unhappy at the idea of the being tied to EU rules for longer.
Mr Bebb, a former defence and Wales Office minister, said: "I think this proposal, which has been floated by Downing Street, is problematic.
"It's problematic because clearly the transition period is 21 months where we will be subject to European Union rules but we will have no say in those rules which is not in my view anything to do with taking back control."
Mr Bebb said the "whole process is creating real damage to the competence of the government".
He added: "These issues are serious and I think the prime minister will have real difficulties persuading the House [of Commons]."
Brecon and Radnorshire Conservative MP Chris Davies, who supported Brexit, said "we're not happy with the continuation of the transitional deal".
But added: "We need to know exactly what this means. Is it going to be another few months and then we're in the same position in a few months' time, in a year's time?
"We need to have a date and we need to know how much it's going to cost the country."