The Registrar said: "CTF Partners has robust compliance procedures in place and is not engaging in activities that would require registration under the Act.
"CTF Partners has provided satsfactory clarification and explicit assurance as to the particular services it does (and does not) offer to clients, as well as full details of compliance training and monitoring materials it has used since the Act came into force."
A Guardian investigation revealed authorities were looking into whether the firm’s work with paying clients overlapped its work with senior Conservatives, including Johnson.
CTF Partners said its staff had volunteered to work on Johnson’s campaign to become Prime Minister by winning an election within the Conservative Party.
The Guardian reports CTF staff worked closely with Conservative MPs – including current ministers – to undermine Theresa May’s Brexit deal. The firm’s founding partner Mark Fullbrook, David Canzini and other senior employees also took leave to run Johnson’s Tory leadership campaign.
In the UK, companies or individuals that lobby ministers and government officials – either verbally or in writing – must list their client and name in the Office of the Registrar of Consultant Lobbyists (ORCL) public register.
CTF Partners always denied it was operating illegally and stressed its employees' work with Johnson was voluntary.
The firm said that as Johnson was a backbencher during the leadership campaign, work on his campaign does not fall under the Lobbying Act.
An earlier Guardian investigation this year found CTF Partners had secretly used Facebook as a tool to spread false information that benefitted clients – tactics that were condemned by the PRCA and CIPR.
CTF Partners is not a member of either body and not bound by their professional codes of conduct.
The PRCA recently cleared Fleishman of any wrongdoing on behalf of its client SMG Europe in Manchester.