Ukraine’s lawmakers are set to ratify a landmark EU association agreement – in proceedings simultaneous with a session of the European Parliament.
Both sides will start the procedure at 10:00 GMT, in what Ukraine’s president said would be a “historic” day.
But the implementation of a free trade deal – part of the package – has been postponed until 2016, apparently under pressure from Russia.
This comes as a fragile ceasefire largely holds in eastern Ukraine.
There were reports of shelling around the government-held airport of Donetsk and other towns.
Pro-Russian rebels have been engaged in heavy fighting with government forces in the Donetsk and Luhansk regions since April – a month after Russia annexed southern Crimea peninsula.
Some 3,000 people have been killed over the past five months of conflict.
Russia denies sending troops and heavy weaponry to help the rebels, as alleged by Ukraine and the West.
In other developments on Tuesday:
- Ukraine’s MPs are to discuss draft legislation aimed at ending the conflict in the east, including a special status for parts of the Donetsk and Luhansk regions
- About 1,300 soldiers from 15 countries – including the US and other Nato members – are continuing military exercises in Lviv in western Ukraine.
Analysis: Laurence Peter, BBC News
The far-reaching EU-Ukraine trade and association agreement at the root of the whole Ukraine crisis is back… but not quite.
The European Parliament and Ukrainian Rada are going ahead with ratification on Tuesday, but the key provisions on free trade will not be implemented until December next year at the earliest.
That delay was the main result of EU-Russia-Ukraine talks in Brussels on Friday, and it is being seen as a significant concession to Russia.
A joint statement after the latest round of trade talks said the parties involved would use the delay to “consult on how to address concerns raised by Russia”.
On the eve of the vote in the parliaments in Kiev and Strasbourg, Ukrainian President Petro Poroshenko said the ratification of the association agreement would be a “historic day”.
Mr Poroshenko – who initialled the deal back in June – said that “de facto this is a reform programme in our country” aimed at guaranteeing “the rule of law, freedom of speech and decisive anti-corruption steps”.
The move is seen by many as a culmination of the country’s recent deadly political upheavals, bringing the former Soviet republic closer to the EU and away from Russia’s sphere of influence.
In November, hundreds of thousands of Ukrainians took to the streets to protest against then President Viktor Yanukovych’s abrupt decision to cancel the agreement’s original planned signing.
However, Mr Poroshenko is facing growing criticism in Ukraine for apparently caving in to pressure from Moscow and postponing the implementation of the free trade deal with the 28-member bloc.
The implementation of the deal – part of the broader agreement – had been originally set for November, but last week was delayed until the end of 2015.
Despite this, the EU said Ukraine would continue enjoying unilateral tariff-free trade with the bloc until then.
Russia has repeatedly warned that once the deal comes into effect, European exports could find their way into Russia duty-free, since Kiev has its own beneficial trade conditions with Moscow. Or less expensive European goods could force Ukrainian goods out of their own market and into Russia.
The Kremlin has also warned it would be forced to introduce protective measures, hurting Ukraine’s struggling economy.
Some politicians in Ukraine reacted with dismay to the postponement.
“I am speechless,” Dnipropetrovsk Region Deputy Governor Svyatoslav Oliynyk was quoted as saying by the AFP news agency.
“The last time this happened, we had EuroMaidan,” he added, referring to the mass protests in November.