The charity, which oversees Scout groups across the UK, has already produced a longlist of potential agency partners and PRWeek UK understands additional applications from agencies will not be considered.
Agencies are being asked to pitch for one or both briefs.
Portland declined to comment on whether it would re-pitch.
The new contracts should be in place around April next year, with a decision likely to be made before the end of the year.
The overall focus is to increase the reach and relevance of scouting, with particular objectives around growing the numbers of adult volunteers and young people joining the Movement. The public affairs brief will focus on engagement with key decision-makers.
There are nearly 620,000 scouts in the UK, made up of 465,000 young people and more than 150,000 adult volunteers. More than a quarter of total membership is female and the waiting lists for young people to join numbers more than 50,000 in total.