“This is a deal which is valid until further notice, but it is an important signal ahead of the coming [Nato] ratification process,” a source with knowledge of the deal told the Aftonbladet newspaper. 

Johnson will meet Sweden’s prime minister Magdalena Andersson at her official country retreat at Harpsund, about 100km west of Stockholm, followed by a joint press conference at 1.40pm.

The two will discuss the security situation in Europe and bilateral relations and cooperation between Sweden and the UK.

According to the newspaper, recent classified meetings of the Swedish parliament’s defence and foreign committees, MPs have been told of secret bilateral cooperation with the UK.

According to several of the newspaper’s sources, Johnson is visiting to officially sign an agreement on this deeper bilateral cooperation, which will, among other things, see British naval vessels dispatched to the Baltic to protect Swedish and Finnish waters during the Nato ratification process. 

Following the press conference, Johnson will then travel onwards to Helsinki, where he will meet with Finland’s president Sauli Niinistö. The goal of that meeting is the same: Johnson and Niinistö will discuss the European security situation and then hold a press conference together.

UK Defence Minister Ben Wallace visited Finland last week, and gave reassurances that the UK would support Finland and Sweden in the event of an attack from Russia, whether or not the two countries decide to join Nato.

This slightly toughened up an earlier pledge of support he gave at a meeting with his Swedish and Danish counterparts at the start of March.