European Union negotiators have a new message for the U.K.: staying in a customs union doesn’t help much if you’re aiming for frictionless trade and minimal border checks.

Brexit negotiations resumed this week in Brussels and on Wednesday the thorny issue of keeping the Irish border open after the divorce is on the agenda. The U.K. has come up with a proposal that would involve staying in parts of the customs union after Brexit as a last-resort way of dealing with the border.

The European Commission, the EU executive, posted a new slide late Tuesday that sets out in color-coded detail all the checks that are still necessary even when a country is in a customs union. Of three long lists of checks and controls, only two items are crossed off when a country is in a customs union.

It’s not very good news for British Prime Minister Theresa May, who has already suffered criticism from hardline Brexit supporters who think the customs compromise risks leaving the U.K. shackled to EU rules for too long.

The slide makes the point that when it comes to border checks, sharing regulation—a feature of the single market—is more important than sharing the same tariffs—a feature of the customs union. The EU negotiating team tweeted and retweeted the slide, which is the latest in a series of color-coded presentations that the EU has released at key moments of the negotiation to make its position clear.

Among the border controls that remain necessary in a customs union are:

  • Live animals
  • Products of animal origin
  • Value-added tax
  • Pelts from countries where leghold traps are used
  • Okra, curry leaves and guar gum from India
  • Plastic kitchenware from China