The zero-tariffs, zero-quotas arrangement has been provisionally applied since January 1 when an eleven-month post-Brexit transition period ended.
But MEPs demanded extra time to vet the pact, which also includes a deal on fishing that was especially difficult to clinch with EU fishing crews losing much of their access to bountiful UK waters.
The European Parliament further delayed its vote in part to protest unilateral delays by London in implementing customs checks in Northern Ireland, one of the most contentious issues in the divorce.
Brussels has launched legal action against London over the Irish problem, while a major row over the supply of UK-based AstraZeneca's Covid-19 vaccine has also embittered cross-Channel relations.
But, despite the acrimony, the European Commission, which handles ties with the UK for the Europeans, has asked MEPs to greenlight the pact, arguing that it will better help keep Britain in check.
The UK, meanwhile, had made it clear that it would not approve any further delays, exposing the risk of the whole deal being annulled if the MEPs did not vote by April 30.
"I assume that the agreement will be ratified almost unanimously," said German MEP Bernd Lange, who heads the parliament's trade committee.
But, he added, "as in sailing, I assume that there will not only be good weather but stormy times as well."
The vote will take place on Tuesday evening in Brussels after a five-hour plenary debate. The result, which is not in doubt, will be announced on Wednesday morning.
– 'Regrettable chapter' –
Britain left the EU on January 30 2020, but its new life with Europe only really began after the transition period on December 31, when London was no longer bound by the bloc's laws and rules.
Officially called a trade and cooperation agreement (TCA), the deal provides for zero tariffs and zero quotas on goods traded between the EU and UK.
But it is less ambitious than many Europeans had hoped for, with nothing on foreign policy and defence nor any commitment to close alignment on environment, health and other regulations.
More harmonised rules would have removed the requirement for some customs checks and paperwork on goods moving between the EU and UK, a new reality that has made doing business more burdensome, with many companies caught unprepared.
In consequence, cross-Channel trade volumes have plummeted, with EU imports from the UK down by nearly 50 percent and exports into Britain down 20 percent in the first two months of the deal's application.
The deal also makes no provision for financial services, threatening the City of London's preeminence as the European hub for capital markets, banking and investment.
Financial ties will be governed by a series of equivalence decisions that are made unilaterally by the EU and require renewal on a regular basis.
Despite ratification, analysts warned that EU-UK relations would remain bumpy, with Prime Minister Boris Johnson eager to show Brexit as a success.
Brussels "clearly has a desire to close this regrettable chapter of European (dis-)integration and relegate the EU–UK relationship to a third-order issue," wrote Fabian Zuleeg and Jannike Wachowiak of the European Policy Centre in Brussels.
"But recent disputes, be it on Covid-19 vaccine supply chains or the implementation of the Northern Ireland Protocol, have demonstrated that both sides' political engagement, and not just technical management, will be needed."