Talks between the UK and the European Union need to "speed up" if a deal on a future relationship can be made in time for Brexit, the EU's negotiator says.
Speaking in Lisbon, Michel Barnier said the UK needed to stop playing "hide and seek" and instead clarify its demands.
It comes as the EU Withdrawal Bill is due to return to the House of Commons, having suffered defeats in the Lords.
The PM faces a rebellion over her move to rule out any future membership of the customs union and single market.
The government fears MPs may follow suit and attempt to amend the bill.
Earlier this week, UK officials warned the EU that its approach to Brexit negotiations risked damaging its security and economic relationship.
Addressing a gathering of jurists in Portugal on Saturday, Mr Barnier called for more clarity on the UK's position, saying an effective negotiation was dependent on knowing what the other side wanted.
He said the EU would be ready to accept movement on Theresa May's "red lines" that insist Brexit must see the UK leave both the European single market and customs union.
"The UK can change its mind," he said, but stressed that "time is tight".
"If the UK wishes to modify its red lines, it will have to tell us so - the sooner the better," he added.
Referencing a row over the UK's potential exclusion from the EU's Galileo project - a multibillion euro plan to build a European GPS system - Mr Barnier said the EU would not be influenced by a "blame game" which seeks to hold the organisation responsible for Brexit's "negative consequences".
The UK said on Friday that it wanted the EU to repay £1bn if it was excluded from the Galileo satellite system,.
"It is the UK which is leaving the EU. It cannot, in the act of leaving, ask us to change what we are and how we function," Mr Barnier said.