I have a 20 litre fuel tank, so always carry cans on a North Sea crossing.
Opinions vary. Some say you can't take any spare diesel in cans. Others say as long as the fuel you have on board in in proportion to what you might need for your passage, then it's OK.
Last time I went to Ostend (earlier this year) in my 25 footer I took diesel in cans. However, we weren't boarded so I never had to put it to the test.
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