Juncker also said during a joint press conference with UK Prime Minister Theresa May in Strasbourg that Brussels would endorse the new documents on Brexit if the deal is supported by the UK Parliament.
"Today, the prime minister and I have agreed on the joint legally binding instrument relating to the withdrawal agreement, this instrument provides meaningful clarifications and legal guarantees on the nature of the backstop […] It complements the withdrawal agreement without reopening", Juncker said at a joint press conference with May.
He also recommended that the European Council should endorse the agreed documents at its meeting on 21-22 March, provided it was subject to a positive vote in the UK parliament.
Juncker also urged British lawmakers to back the Brexit withdrawal bill on Monday after agreeing on enhanced legal guarantees with May. "The choice is clear. It is this deal, or Brexit may not happen at all. Let's bring the UK's withdrawal to an orderly end", Juncker said.
In turn, May said in Strasbourg on Monday that in addition to a joint instrument and a joint statement announced earlier in Parliament, Britain would also make a Unilateral Declaration on the so-called backstop — an insurance policy aimed at avoiding a hard border on the island of Ireland.
While London managed to negotiate a withdrawal deal with Brussels after months of intense talks, the agreement has faced a wave of criticism in the United Kingdom and spurred mounting calls for a second Brexit referendum, with Parliament so far refusing to endorse the deal due to the controversial provision on the Irish border backstop.
The UK House of Commons is set to hold a vote on the government's Brexit plan on Tuesday. If the government's deal does not pass then, a vote on a no-deal scenario will be reportedly held on Wednesday. If that option is rejected as well, a vote on a Brexit extension is expected on Thursday.
The United Kingdom is set to leave the European Union on 29 March.