OSLO (Reuters) - British citizens already living in Norway and Norwegian citizens living in Britain will have the right to remain residents, even in case of a no-deal Brexit, the prime ministers of Britain and Norway said on Tuesday.

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The agreement announced Tuesday was the first concrete step agreed between Britain and the Nordic country on terms that would apply after Britain leaves the EU in March. Norway is not a member of the EU but is part of the single market as a member of the wider European Economic Area (EEA).

“Prime Minister May and I agreed that Norway and UK will put in place a comprehensive citizens rights’ agreement,” said Erna Solberg, prime minister of Norway.

“We will treat all UK citizens living in Norway ... so they will have the same opportunities as they had before also after March 2019,” she said, adding that Britain and Norway were “very close” on agreeing a deal to mirror any Brexit deal London concludes with Brussels.

British Prime Minister Theresa May, visiting Oslo, said she was making the same commitment to Norwegian citizens, as part of a wider pledge to grant such rights to citizens of all EEA countries already living in Britain.

“Whatever happens, we confirm that people from the EEA, the Norwegian citizens and those others who are living in the UK, and who have made their life choice to be in the UK, well, to be able to be in the UK. We want them to stay.”

Solberg said that in the event Britain leaves the EU without a free trade deal with the EEA countries, the most challenging issue between Norway and Britain would be the trade in goods.

“The most difficult part will be goods, especially from Norway to Britain, because there will be problems on the British side more than on our side,” she told Reuters.

“We will have to deal only with Britain, but (Britain) will have to deal with everybody,” she said after a session of the Nordic Council at the Norwegian Parliament where May spoke earlier.

Britain is Norway’s most important trading partner, buying oil, gas and fish.

Still, Solberg said she “absolutely believed” that Oslo and London would be able to make “things function” between Norway and Britain even in the case of a hard Brexit.