Political lobbyists could be fined if they fail to sign-up to a planned register of those working in the industry, the government revealed on Friday.

But a consultation launched by ministers, due to run until early April, is expected to focus closely on what the definition of a lobbyist should be. In launching the consultation the government says: "Lobbyists should be defined as ‘those who seek to influence or change government policy on behalf of a third party’".

They are basing their ideas upon a similar register which has existed in Australia for several years, but the expectation is that union leaders will be included in this broad definition of lobbying.

Minister also think the lobbyists should declare whether they used to work for the government in any way. It is very common for Whitehall and Parliament staff to move into lobbying later in their careers.

Cabinet Office minister Mark Harper says: “We already publish an unprecedented amount of information about who Ministers and senior officials meet. However, it’s not always possible to understand the significance of these meetings, because it’s not always obvious who the people ministers and officials meet represent."

But the RMT union leader Bob Crow was scathing of any attempt to include union barons in the register: "The idea that trade unions, representing millions of workers up and down the country, should be bracketed in with the chancers and shmoozers from the shadowy world of political lobbying is a gross insult to men and women fighting for a fair deal in the workplace.

"This is just another blatant ConDem attack on the trade union movement and shows complete and utter contempt for the role we play in protecting working people from the savagery of casino capitalism."

Executives of one firm, Bell Pottinger, including an ex-Tory politician, were recently secretly taped saying they could directly influence Cameron and senior ministers on behalf of private sector clients.

Downing Street dismissed that as "simply untrue" but the case further fuelled demands for a register following a string of scandals over recent years.

In 2010 three former Labour ministers were stripped of parliamentary passes after they breached lobbying rules and the House of Lords was rocked by a "cash for amendments" scandal.

Stephen Byers, a former trade secretary, was caught by undercover reporters posing as a lobbying firm describing himself as “like a sort of cab for hire” for up to £5,000 a day to use his contacts to push for changes in policy.

Concerns over the influence being wielded by lobbyists also led to the resignation of defence secretary Liam Fox.

There are rules on political lobbying, but there is currently no law that compels lobbying firms to register their activity.

Labour said any new system needed to include a detailed code of conduct with "clear consequences for those who breach it", including being struck off in the worst cases.