She said: 'We will not accept an agreement that gives the European Union a lock on ending the backstop when we have new customs arrangements in place that meet our commitments on avoiding a hard border for Northern Ireland'.
Addressing a news conference in Brussels on Friday, Barnier said there was a risk of a "blame game" emerging with both the United Kingdom and European Union at risk of blaming each other in the event of the Brexit talks not progressing at the desired pace.
Under its "backstop" proposal, there would be no tariffs, quotas, rules of origin or customs declarations on all UK-EU trade, not just trade between Northern Ireland and the Republic of Ireland.
The inclusion of the December 2021 date in the latest government document on the Temporary Customs Arrangement, as it has become officially known, is thought to have been a response to demands for a clear time limit from Brexit Secretary David Davis. It would have signed us up to the rules of the EU Customs Union (a central pillar of Brussels rule) if it was ever activated.
PM May also faces a series of votes in the House of Commons on Tuesday 12, June on whether to approve 15 "Brexit wrecking" amendments inserted into the EU Withdrawal Bill by the House of Lords.
"We are today at a time of decisions and choices".
She said: "This is about peace, not Brexit".
"We are not going to be intimidated by this form of blame game", Barnier told a press conference in Brussels.
He said: "The clock is ticking on the Brexit process and it is imperative that an agreement is struck which will protect jobs, the economy and ensure there is no hard border in Northern Ireland".
The most contentious changes center on Britain's future economic relationship with the EU.
Whitehall has proposed two customs solutions: a partnership under which Britain would collect import duties on the EU's behalf, and a proposal known as "max fac" (maximum facilitation) that would rely on new technology to conduct rapid border checks without a physical presence at the historically fraught frontier.
Yesterday's agreement is created to provide a "backstop" - a guarantee of no hard border in Northern Ireland should other solutions fail. Barnier said he and his team are still studying Britain's offer, but that London appears to want to keep the whole of the U.K.in parts of the customs union.
The group's chair Mark Malloch Brown acknowledged a second poll could confirm the Leave vote, and pledged that Best For Britain would respect the result whichever way it went.
Migrants gathered outside the Asylum Seekers Centre in Tirana said they wanted to find work in a variety of European Union countries, dodging police on the way there. "I reject nothing. I ask questions". We believe this is the right thing to do so that everyone's voice can be heard.
Only 46.7% said that they would identify as British.
Over the past 24 hours there were veiled threats that the Secretary of State for Exiting the EU, David Davis, might resign if a deadline was not set on the extension of customs arrangements.
- G7 leaders set to clash with combative Trump over tariffs, trade
- What you need to know as you head to the polls
- Stephens downs Keys again to reach French Open final
- Cavaliers let chances slip away _ again _ in NBA Finals
- Canada co-ordinating tariff attack on United States with EU
- Danone joins fight against plastic waste in New Zealand
- How To Get iOS 12 Right Now
- Sloane steady wins the race for Stephens
- How to get help for someone who might be suicidal
- Hot-Shooting Curry Leads Warriors To NBA Title