It also found that "lobbying is dominated by the corporate world" with 80 per cent of the most frequent lobbyists coming from FTSE 100 companies.
Transparency International said that 73 MPs earned a total of £3.4m last year for external advisory roles, an arrangement that created a "significant risk of conflicts of interest".
Nick Maxwell, head of research at Transparency International UK, said: "Lobbyists attempt to influence decisions that affect billions of pounds of public spending and millions of lives across the UK. Greater transparency about lobbying can help build confidence that the Government is working in interests of citizens, rather than lobbyists. The current lobbying register is entirely unfit for purpose, with less than four per cent of lobbyists being covered by the register."
As of today, 98 lobbying firms are signed up to the Government’s register.
Phil Morgan, deputy chief executive of the CIPR, said: "The Government does not think it has a problem around the transparency of in-house lobbyists because ministers’ diaries show who they have met, but in-house lobbyists can talk about any number of issues rather than just the core business they represent. Ministers’ diaries are not published often enough and do not contain detailed information. The public has a right to know, which is not being met."
Morgan said registers were important but that governing the behaviour of lobbyists with a code of conduct, included in the CIPR’s register, was a greater priority.
He said: "Most lobbyists operate in plain view but there is a lingering problem with the perception of some. The public needs to have confidence that lobbyists are acting professionally."
Iain Anderson, chairman of the APPC, said: "The Governments lobbying legislation is not fit for purpose and it needs fundamental reform. Government needs to talk outside the Westminster bubble to business, unions and third sector. The best way to address all of this is to publish ministerial diaries and be done with it."
The Office of the Registrar of Consultant Lobbyists, which operates the statutory register, was contacted but was unavailable for comment at the time of publication.