While Sweden's ruling Social Democrat-Green coalition remains against joining Nato, support for membership has been growing in Sweden in the past year – largely credited to a rising fear of Russian military activity in the Baltic region.

But the centre-right Alliance opposition has up until now been split on the issue with the largest party – the Moderates – and the Liberals being strongly in favour of Nato and its partners the Centre Party and the Christian Democrats taking a more hesitant stance.

However, in what spells a significant U-turn, Centre Party bosses announced on Tuesday that they would motion their colleagues to advocate membership in the defence organization at the party's next conference.

“We lack the ability to defend ourselves for a longer period of time. At the same time Nato is very clear about the fact that Sweden cannot expect military support if we are not full members of the organization. We can no longer close our eyes to that,” wrote party leader Annie Lööf together with foreign policy and defence spokespeople Kerstin Lundberg and Daniel Bäckström in an opinion piece for the Svenska Dagbladet newspaper.

“We fear that false non-alliance risks becoming a security risk rather than offering safety,” they added, referring to Sweden's increased military cooperation with its neighbours despite officially maintaining a semi-neutral position.

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Meanwhile, the Christian Democrats, who have previously opened the door to the possibility of joining Nato, are also set to debate the issue at their party conference in early October.

“We are a relatively small country with a strategically important location in the Baltic Sea region. Our assessment is that we can't go it alone but need to cooperate with others to manage our defence capability and we need to do that as part of Nato,” Christian Democrat leader Ebba Busch Thor told the TT newswire on Tuesday.

Sweden's ruling centre-left coalition – the Social Democrats and the Green Party – is historically against Nato membership. However, there have been indications in the past year that the Nordic country is moving closer to joining the defence alliance.

In April Sweden, Norway, Denmark, Finland and Iceland announced far-reaching plans to extend their military cooperation. The move was “a direct response to aggressive Russian behaviour”, said Sweden's defence minister Peter Hultqvist and his Nordic counterparts at the time.

Although Sweden is set to invest 10.2 billion kronor into its armed forces in the coming year, the country's defence capabilities have been questioned following increasing military activity from Russia in the Baltic region.

In October 2014, a foreign submarine – suspected to be from Russia, although this was never confirmed – was spotted in Swedish waters just outside Stockholm. A number of Russian planes have also been spotted in or close to Swedish airspace over the past year.