The risk that Russia could simply sell its oil, which is affected by an EU embargo, elsewhere in the world has occupied the European Commission for weeks.
In the sixth package of sanctions that has now been presented, it is therefore trying to at least make this more difficult.
Tankers flying the flag of an EU member or operated by a shipowner based in the EU should no longer be allowed to transport Russian crude oil and refined products.
Henrik Kafsack
Business correspondent in Brussels.
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In addition, the Commission wants to ban all services required for ship transport, such as financing or brokerage services.
Above all, it should no longer be possible to insure oil tankers in the EU against environmental catastrophes and other damage.
Without such insurance, the ships used to transport oil are effectively worthless.
The costs of an oil spill can quickly reach billions.
How effective that will be is hard to say.
The American government had tried to prevent the Nord Stream 2 pipeline with very similar methods, but - also thanks to German help to circumvent this - without success.
Ultimately, the decisive factor is the extent to which other states join the EU.
Finally, countries like Liberia also have large shipping fleets.
The leading insurance exchange Lloyd's is based in London.
The EU can hope that the United Kingdom will not undermine its embargo.
But there is no guarantee.
Accordingly, immediately after the Commission's proposal was presented, the three EU member states with the largest shipping fleets, Greece, Cyprus and Malta, expressed major concerns, according to diplomats.
They argued that the cargo ban would make no sense without close international coordination.
Cyprus and Greece had already watered down the berthing ban for Russian ships in EU ports in the fifth sanctions package.
However, diplomats from other EU countries are confident that they can still be persuaded to agree.