Nato secretary-general Jens Stoltenberg said that Finland and Sweden have officially applied to join the world’s biggest military alliance, in a move driven by security concerns over Russia’s war in Ukraine.

“I warmly welcome the requests by Finland and Sweden to join Nato. You are our closest partners,” Mr Stoltenberg said after receiving application letters from the two Nordic countries’ ambassadors.

“This is a good day at a critical moment for our security.”

The applications must now be considered by the 30 member countries.

That process is expected to take about two weeks, although Turkish President Recep Tayyip Erdogan has expressed reservations about Finland and Sweden joining.

If his objections are overcome, and accession talks go as well as expected, the two could become members within a few months.

The process usually takes eight to 12 months, but Nato wants to move quickly given the threat from Russia hanging over the heads of the Nordic countries.

Canada, for example, says that it expects to ratify their accession protocol in just a few days.

Mr Stoltenberg said that Nato allies “are determined to work through all issues and reach rapid conclusions”.

“All allies agree on the importance of Nato enlargement. We all agree that we must stand together, and we all agree that this is an historic moment which we must seize,” he told reporters at Nato headquarters in Brussels.

The fact that the Nordic partners applied together means they will not be losing time by having to ratify each other’s membership application.

“That Sweden and Finland go hand-in-hand is a strength. Now the process of joining the talks continues,” Swedish foreign minister Ann Linde told the Swedish news agency TT.

It should not take long to win approval in the Baltic states of Estonia, Latvia and Lithuania. Their prime ministers issued a joint statement on Wednesday saying that they “fully endorse and warmly welcome the historic decisions” taken in Helsinki and Stockholm.

Public opinion in Finland and Sweden has shifted massively in favour of membership since Russia invaded Ukraine on February 24.

Finland and Sweden co-operate closely with Nato. They have functioning democracies, well-funded armed forces and contribute to the alliance’s military operations and air policing.

Any obstacles they face will merely be of a technical, or possibly political nature.

Nato’s membership process is not formalised, and the steps can vary. But first their requests to join will be examined in a sitting of the North Atlantic Council (NAC) of the 30 member countries, probably at ambassadorial level.

The NAC will decide whether to move towards membership and what steps must be taken to achieve it.

This mostly depends on how well aligned the candidate countries are with Nato political, military and legal standards, and whether they contribute to security in the North Atlantic area. This should pose no substantial problem for Finland and Sweden.