Sweden is likely to sign an agreement allowing Nato forces to deploy, reports newspaper Svenska Dagbladet.
Peter Rådberg, defence spokesperson for the Greens, told the paper the government should have discussed the idea before deciding. And Left Party leader Jonas Sjöstedt told Swedish Radio, in a debate with Foreign Minister Carl Bildt, that the government is aware that the Swedish people - if asked if they wanted to join Nato - would have voted no.
"That is why the government chooses another strategy. They make us a kind of de-facto-member in area after area, in order to say that 'now we are almost members of Nato, now the only thing missing is the influence and article 5'. I think this is a dishonest strategy and it damages our non-alignment which is important for an independent foreign policy," said Sjöstedt.
(Article 5 of the Nato treaty requires member states to come to the aid of other member states if they are attack).
After newspaper Svenska Dagbladet broke the story, defence minister Karin Enström confirmed to Swedish Radio News that the government is going to announce on Thursday that they have decided to become a Nato host nation.
Sweden is already linked with Nato via another 20-year-old agreement, which has seen Swedish forces in Afghanistan and Kosovo. Nato has also held exercises here.
But a new agreement would mean Nato could quickly get support from Sweden, and use resources or be stationed in the country, if the government invited them.
According to Foreign Minister Carl Bildt, this is not a way to 'sneak Sweden into Nato', it is normal co-operation between the Nordic countries and Nato so that they can carry out joint exercises. During the radio debate with the Left Party leader, Bildt said: "We are not sneaking in, this has support in the parliamentary defence committee."
Bildt thereby tried to put focus on the split between the opposition parties in this issue. Because despite the Greens and the Left objecting, the biggest opposition party, the Social Democrats, are in favour of moving closer to Nato, although not outright membership.
So with the help of the Social Democrats, the government's proposed Nato agreement is expected to pass when it is put to a vote in parliament.
This move comes as Finland are also looking to sign a similar agreement, as Nato prepares for a summit meeting in the UK, in September.
The agreement would be a deepening of the current Nato-Sweden cooperation, under the "Partnership for Peace."
Since 2002 Sweden has moved from neutrality, to being ready to offer military help to EU-countries, and in 2013 all parties (apart from the Left Party) agreed to develop the possibility for Sweden to give and accept military help.