Romania and Bulgaria took a significant step toward full integration with the European Union on Sunday as they partially joined Europe's ID-check-free travel zone.

This milestone, following years of negotiations to enter the Schengen Area, now grants travelers arriving by air or sea unrestricted access.

However, land border checks will persist due to opposition, notably from Austria, citing concerns over illegal migration.

EU Commission President Ursula von der Leyen celebrated this development as a "remarkable achievement" for both countries, marking a "historic moment" in the world's largest free travel zone.

The Schengen Area was established in 1985. Before Bulgaria's and Romania's admission, it comprised 23 of the 27 EU member countries, along with Switzerland, Norway, Iceland and Liechtenstein. Around 3.5 million people cross an internal border each day.

Austria vetoed Romania and Bulgaria's admission into the Schengen Zone at the end of 2022 but allowed Croatia full accession. Bulgaria and Romania joined the EU in 2007 and Croatia joined in 2013.

Siegfried Muresan, a Romanian Member of the European Parliament, said that it is "an important first step" that will benefit millions of travelers annually.

"Bulgaria and Romania have been fulfilling all criteria for joining the Schengen Area for years - we are entitled to join with the terrestrial border as well," he said, adding that it "will offer additional arguments to the last EU member state that has been vetoing the full accession."

Romanian Prime Minister Marcel Ciolacu called it a "well-deserved achievement" for Romania, saying it would benefit citizens who could travel more easily and would bolster the economy.

"We have a clear and firmly assumed government plan for full accession to the Schengen Area by the end of the year," he said.

The EU's executive branch, the European Commission, has said for more than a decade that Romania and Bulgaria both meet the technical criteria for full accession, which requires unanimous support from their partners. Both countries have agreed to implement random security screening at airports and maritime borders to combat illegal migration and cross-border crime.

While lifting border controls on air and sea ports is expected to positively impact the tourism sector, members of the European Parliament have voiced concerns about long queues at the EU's land borders and the impact they can have on trade in the bloc's single market, as well as the health and safety of drivers.

Truck drivers are frequently stuck in kilometers-long queues at the borders of both Romania and Bulgaria. The Union of International Carriers in Bulgaria estimates delays cost the sector tens of millions of euros each year.