Amid criticism of the deal from lawmakers on all sides of Britain's EU debate, European Commission chief Jean-Claude Juncker said the United Kingdom couldn't hope to negotiate better terms before its departure in March.
"Our key finding is that if the Government's proposed Brexit deal is implemented so that the United Kingdom leaves the EU Customs Union and Single Market in 2021, then by 2030 GDP will be around four per cent lower than it would have been had the United Kingdom stayed in the EU", the report said.
Lithuanian President Dalia Grybauskaite said if the UK Parliament rejected the deal, there could be an extension of the negotiations or another referendum.
After the rubber stamping, all eyes turn to British Prime Minister Theresa May to sell the deal to her recalcitrant parliamentarians over the next days and weeks - a huge task considering the intense opposition both her Conservative lawmakers and the political opposition have voiced toward the deal.
Under the deal secured with European Union leaders on Sunday, Britain will leave the bloc in March with continued close trade ties, but the odds now look stacked against May getting it approved by a divided British parliament.
The deal, which was the result of a year and a half of often contentious negotiations, still needs to be approved by the British Parliament, where a politically diverse majority of lawmakers are already lined up against the deal for being too much, too little, or totally stupid and unnecessary.
"We can not go on any more with this hopeless can't-do spirit; this reflexive negativity and defeatism that so woefully underestimates the courage and creativity of the British people", he lamented.More news: Trump's asylum restrictions blocked by federal judge
Mrs May added: "This deal avoids a hard border between Northern Ireland and Ireland".
In the highly likely scenario that a EU-UK trade deal is not close to ratification by July 2020, the EU and UK will jointly decide at this so-called "rendezvouz" point, whether to extend the transition period for up to two years, again precluding such a deal with Washington.
What will happen to the withdrawal agreement if it is voted down by the British Parliament is unclear.
The Bank of England, Treasury, and research houses are all making last-minute efforts to provide quantifiable evidence to show Brexit's economic impact before the vote, which has been set forDec. 11. May says "it is the best possible deal".
"We say (Article 50) can be unilaterally revoked - whether that's the case or whether the court just says it can be revoked with permission - we've got the answers that we need in order as members of parliament to be able to say there is a concrete option of stopping Brexit".
He added: "It's not the intention of the Prime Minister, the Cabinet, nor of the Parliament to go for a second referendum". He argued that they are seriously affected by their impending irreversible loss of European Union citizenship and its associated rights due to the referendum vote and ensuing negotiations in which they had no say.
More than an hour passed before the first member of her own party stood up to voice her support for the prime minister.