Atherton Associates joined the APPC last week, taking membership to 43 firms. The agency was set up by former Labour MP Candy Atherton after she lost her seat to Liberal Democrat Julia Goldsworthy in the 2005 general election.
Meanwhile, five-year-old agency JMC Partners has also joined. Last month, APPC signed up B2L Public Affairs and Positif Politics.
APPC chair Gill Morris said the flurry of firms signing up showed ‘the value of the code and strengthening the UK's self-regulatory regime'.