Shortcomings in the EU's attempts to return unsuccessful asylum-seekers are "encouraging illegal migration", the bloc's watchdog said in a report on Monday.
The European Court of Auditors, which scrutinises EU policies and finances, found efforts by Brussels to gain co-operation from countries outside the bloc to take back migrants "yielded limited results".
Fewer than one in three migrants ordered to leave the EU actually do, and that ratio drops to less than one in five when the country to which they are told to return is outside the European continent.
The ECA's audit of the policy between 2015 and mid-2020, which focused on the 10 countries with the biggest numbers of non-returned irregular migrants, said the steps being taken to improve co-operation were "relevant".
But they fell short in areas including data-sharing between EU member states. Members' agencies cannot even agree on how many irregular migrants are ordered to leave, the report said.
The EU statistics office Eurostat, on which the ECA relies, says that about 500,000 migrants a year are ordered to leave the bloc.
But Frontex, the EU border guard service that also helps with returning migrants, uses a different method to arrive at a lower figure.
In 2019, Frontex counted 298,190 return decisions, compared with Eurostat's figure of 513,470.
Leo Brincat, the lead author of the ECA report, said that the accumulation of such shortfalls hampered the goal of swiftly returning irregular migrants.
"Rather than discouraging, they end up actually encouraging illegal migration," Mr Brincat said.
"It is well known among the migrants that the returns are not effective, so this can actually encourage them to come even more."
Irregular migration is one of the most pressing issues for the EU since the 2015 inflow of asylum seekers largely fuelled by the war in Syria.
But an attempt to overhaul migration and asylum rules across the bloc has reached an impasse.
Eastern member states in particular have rejected its core principle of sharing the hosting of asylum seekers so frontline countries such as Greece and Italy are not alone in dealing with it.
The issue is complicated by the fact that, while Brussels is meant to come up with rules and legislation in this area, member states retain responsibility over return decisions and how they are carried out.
Since the early 2000s, the EU has reached agreements with 18 countries outside the bloc, including Turkey, Albania, Serbia and Pakistan.
It has opened talks with six others, including China, Nigeria, Morocco and Algeria. Several EU countries have their own, bilateral treaties with some countries.
But the ECA report found countries are baulking at EU insistence that they take back migrants who travelled through their territory, as well as their own nationals.
It said that non-binding arrangements, such as those negotiated between 2016 and 2018 with Afghanistan, Bangladesh, Guinea and Ethiopia, had proved more successful.
The ECA recommended that the commission take "a more flexible approach when negotiating readmission agreements".
Incentives should be strengthened to coax countries to go along with such agreements with the EU, which already happens by linking visas to compliance, it said.
And member states should boost co-operation between themselves to reduce the number of irregular migrants absconding before their ordered return to their country, and improving data on returns.