A diplomatic offensive is underway to ensure the U.K. does not discriminate against any EU country with its post-Brexit work visa fees, which are more expensive for the nationals of five member states than for the rest of the bloc.

Following the end of free movement, European Economic Area nationals must apply for a work visa in order to take up a job in Britain. But not all of them pay the same.

The citizens of 25 countries — including 21 in the EU — are subject to a £55 discount in fees when applying through several visa routes, including those targeted at entrepreneurs, health care staff, researchers, charity workers, medium-skilled workers and temporary workers such as fruit pickers. Employers also save money when recruiting from these countries, as companies are not asked to pay the £199 fee needed to issue a certificate of sponsorship to hire them.

Irish citizens do not need work visas because of the Common Travel Area with the U.K., but five EU countries are not eligible for reduced fees: Bulgaria, Estonia, Lithuania, Romania and Slovenia.

Some of these EU countries are now demanding help from the European Commission to tackle the difference, arguing Brussels cannot allow the U.K. to breach the EU’s long-standing principle of non-discrimination among member countries.

“In our view, this is a differentiated treatment that needs to be carefully scrutinized,” one diplomat said. “The issue is not about the additional £55 that the five member states citizens will need to pay compared to other EU citizens, but clearly about the differentiated treatment that they are faced with.”

“People across the EU will feel they are being treated differently and that will raise lots of questions,” a second diplomat said.

The British government argues its eligibility list is based on the signatories of the Council of Europe’s Social Charter (CESC), an international treaty dating back to 1961 adopted by 26 countries including the U.K.

Article 18.2 of the charter sets an obligation for signatory countries to “simplify existing formalities and to reduce or abolish chancery dues and other charges payable by foreign workers or their employers.”

But the EU diplomats question the U.K.’s interpretation of this charter, saying not all the EU countries who signed it declared themselves bound by Article 18.2, yet many are still eligible for reduced fees for U.K. visas. That is the case for Croatia, the Czech Republic, Finland, Hungary, Latvia, Malta and Portugal, among others.

The diplomats also counter that all five EU countries not currently eligible for reduced fees ratified a revised charter at the turn of the century, which also includes the obligation to cut or abolish visa fees. “These are two instruments with basically the same aim that exist in parallel,” the first diplomat said.

However, the U.K. government insists its policy is based on the original treaty of the Council of Europe, and argues that reducing visa fees for any country where this is not related to an international obligation would itself raise questions of discrimination.

“Visa, immigration and citizenship fees are set at a level that helps provide the resources necessary to operate our border, immigration and citizenship system,” a spokesperson for the country's Home Office said. “In fairness to U.K. taxpayers, it is only right that those who directly benefit from our immigration system contribute to its funding. We will be considering visa fees in the round as part of our longer-term review of funding the immigration system.”

The EU envoys are piling pressure on the Commission, saying this is the first example of the U.K.’s post-Brexit immigration rules treating EU nationals differently on the basis of their nationality. They warned post-Brexit Britain is likely to discriminate between EU citizens in other looming mobility decisions, for instance, when it decides whichcountries can participate in its Youth Mobility scheme — currently open to nine developed economies outside the bloc.

A spokesperson for the Commission said that while the EU and the U.K. are free to determine their respective visa policies, “the U.K. has committed in the Trade and Cooperation Agreement to treat all nationals of EU Member States equally for the purposes for the purposes of short-term visas” and “cannot decide to grant a visa waiver for short-term travel to citizens of certain Member States, whilst excluding others.”

However, it may be hard for Brussels to find a legal basis to compel London to address the difference of treatment when it comes to long-term visas, which are not covered by the Brexit deal. “We regret that the U.K. decided to treat EU citizens differently in this specific case,” the spokesperson said.

Unresolved mobility issues are likely to be decided bilaterally between the U.K. and EU national governments — potentially leading to a patchwork of differing rules across the bloc.

“In a couple of years’ time, depending who has negotiated what, different EU citizens will have different access to different programs and at a different cost,” a third diplomat said.

The same diplomat added: “The reduced visa fees controversy is just the first issue of a number of them which are simmering beneath the surface but will come to the fore sooner or later. It will be interesting to see if there’s going to be EU-wide coordination on this or not. It all goes back to: Are we all the same or suddenly we are not? Are we going to live up to EU solidarity? Are we going to stick together? There’s no EU competence on this so it throws the ball back to member states.”