The European Court of Justice will issue its verdict on a legal challenge that Hungary and Slovakia lodged against the scheme.
The European Court of Justice will issue its verdict on a legal challenge that Hungary and Slovakia lodged against the scheme that requires bloc members to take in Syrians and others from overstretched Greece and Italy.
The European Union approved the scheme as part of efforts to boost EU solidarity and end the chaos generated by Europe's worst migrant crisis since World War II.
The Luxembourg-based court's senior lawyer Yves Bot recommended in July that the judges throw out the challenge, arguing the so-called relocation scheme was a proportionate means to help Greece and Italy.
The court often follows such opinions but not always.
At stake is the EU's legal authority to take joint action to ease an unprecedented crisis and override opposition from a minority of member states.
In this case, eastern member states rejected the plan on grounds they lack the capacity to integrate foreigners, most of them Muslim.
No matter the outcome of the court case, the scheme has been troubled from the start.
EU figures show that just under 28,000 people have been relocated since a majority of member states agreed in September 2015 to relocate 160,000 Syrian, Iraqi and Eritrean asylum seekers from Italy and Greece by September this year.
Officials in Brussels have argued the scheme is legally binding on all member states, including those who voted against the quotas like Hungary, Slovakia, the Czech Republic and Romania.
Under the plan, Hungary must admit more than 2,300 asylum seekers, while Slovakia must in the long term take in 1,400.
But in June, the European Commission, the 28-nation EU executive, launched legal action against Poland and Hungary for having failed to admit any asylum seekers.
It also took legal action against the Czech Republic for having stopped taking them but spared Slovakia which agreed to take a handful of them.
Although it voted for it, Poland has come out strongly against the plan since a right-wing government came to power.
The EU action -- which would be bolstered by a favourable ruling in Luxembourg -- could see the three countries referred to the Court of Justice and given stiff financial penalties.
Other EU member states have dragged their feet despite having voted for the plan.
In June, Filippo Grandi, the UN's High Commissioner for Refugees, called the relocation plan a "disappointment" where only a handful of countries on a wealthy continent were fulfilling their duty to asylum seekers.
He singled out Greece, Italy, Germany, Sweden and Austria.
European sources have blamed the delays on governments trying to screen jihadists in the wake of terror attacks, a lack of housing and education for asylum seekers, and logistical problems.
With the relocation scheme struggling to lift off, many Syrians and others in Greece and Italy have travelled with smugglers to wealthier northern countries like Germany and Sweden.
Brussels launched the relocation scheme in September 2015, the year more than one million migrants arrived in Europe by sea.
It was introduced as an exception to so-called Dublin rules under which migrants must apply for asylum in the member state where they first land.
Under international and European law, countries are required to grant asylum to people fleeing war or persecution but not those classed as economic migrants.
Political pressures have eased since March last year when the EU signed a deal with Turkey to send back migrants in return for billions of euros in aid and for admitting asylum seekers directly from refugee camps in Turkey.
The controversial deal sharply cut the flow across the Aegean Sea to Greece, which had been the main point of entry to Europe.
Other routes have since seen a resurgence in migrants, including via Libya to Italy as well as Morocco to Spain. The EU deems most people coming from sub-Saharan Africa as economic migrants.
More than 14,000 people have died attempting to reach Europe since 2014.