David Cameron has been accused of cynically exploiting the latest lobbying scandal to hit Westminster by using the opportunity to undermine trade union support for Labour.

The Prime Minister’s official spokesman said the Government would couple plans to limit the amount of money which unions could spend at general elections with moves to bring in a statutory register of lobbyists. They would also force unions to undergo annual audits on the size of their membership.

But within hours of the announcement, senior Liberal Democrats claimed the proposed anti-union legislation had not been agreed by Nick Clegg and distanced themselves from the official Downing Street briefing.

Conservative sources had insisted the plans represented Coalition policy – a claim angrily denied by the Lib Dems. “None of what they told you has been agreed in government,” said a senior source. “We will not be part of any sort of grubby political deal to attack the unions.”

The proposed Bill also provoked fury among union leaders and was denounced by Labour as a “shabby and panicked” response to fresh accusations of improper lobbying. Frances O’Grady, the TUC general secretary, said: “The Government is cynically trying to exploit a political sleaze scandal to crack down on unions.”

The allegations have resulted in the suspension of two Labour peers from the party and the resignation of an Ulster Unionist peer from his party. The MP Patrick Mercer has also resigned the Tory whip over accusations he agreed to be paid in return for Fiji to be readmitted to the Commonwealth. All the parliamentarians deny any wrongdoing.

The row erupted as new figures revealed that major companies – including financial institutions, internet giants and car manufacturers – are paying hundreds of thousands of pounds to Westminster special interest groups. They have spent more than £600,000 in support for just eight parliamentary all-party groups, an analysis by The Independent has established.

It was reported last night that 80 parliamentary passes issued by all-party groups had been revoked by the Commons authorities after it was discovered 40 were held by lobbyists.

The proposed lobbying register announced by Downing Street will list companies which lobby on behalf of clients. Legislation would be published within six weeks.

But the Prime Minister’s spokesman also set out plans to curb union spending at elections. He said they were aimed at organisations which are affiliated to a political party or make donations of more than £100,000. During elections the full expense of their help will have to be declared and count towards the limit on spending by a party.

Meanwhile, there were calls for the Government to require greater transparency from all-party parliamentary groups to answer charges they have become discreet vehicles for the lobbying of parliamentarians. Mr Mercer promised a fictitious lobbyist that he would set up a parliamentary group to promote the interests of  Fiji.

Supporters of the Parliamentary Internet and Communications Technology Forum, which updates Westminster on developments in the digital media, include Motorola and Vodafone. Volvo and Michelin Tyres are among the companies subsidising the all-party transport safety group.

Tamasin Cave, spokeswoman for the Alliance for Lobbying Transparency, said: “Many of these groups are just interest clubs for MPs, but some are significant lobbying outfits, operated and directed by commercial lobbyists.”

Money trail: Westminster’s special interest groups

All-Party Parliamentary China Group

Chair: Richard Graham (Con). Vice-chairs include Lord Steel (Lib Dem)
Remit: To “engage with the People’s Republic of China (including Hong Kong) and to develop all aspects of the bilateral relationship”. Arranged trip to Hong Kong, paid for by Hong Kong government.
Income: £48,574.
Supporters: GKN Group Services (£10,000), HSBC (£10,000), City of London (£5,000)

All-Party Parliamentary Group for Trade and Investment

Chair: Margot James (Con). Vice-chairs include Pat McFadden (Lab)
Remit: “To research and inform debate over how best to support British exporters … and encourage foreign direct investment into the UK”.
Income: £84,000
Supporters: Barclays (£40,000) and Prudential (£40,000)

Parliamentary Advisory Council for Transport Safety

Co-chairs: Jim Fitzpatrick (Lab), Sir Peter Bottomley (Con), John Leech (Lib Dem)
Remit: Registered charity which updates Parliament on air, rail and road safety issues; to “protect human life through the promotion of transport safety for the public benefit”.
Income: £66,050.
Supporters: Volvo (£17,192), Direct Line Group (£5,000), Michelin Tyres (£3,400).

All-Party Parliamentary Corporate Governance Group

Chair: Andrew Tyrie.
Remit: To promote a “culture based on responsible leadership and investment, thus enabling corporates and their shareholders to enhance prosperity for the benefit of shareholders.
Income: £62,000.
Supporters: Lloyds TSB (£10,000), Standard Life (£7,500), Old Mutual (£5,000)

All-Party Parliamentary Beer Group

Chair: Andrew Griffiths (Con) Vicechairs include Dan Rogerson (Lib Dem)
Remit: To promote the wholesomeness and enjoyment of beer and the unique role of the pub in UK society
Income: £65,849
Supporters: Molson Coors (£8,227), Greene King (£6,227) and Carlsberg (£3,000)

All-Party Parliamentary group for the Armed Forces

Chair: James Gray (Con). Vice chairs include Bob Ainsworth (Lab)
Remit: “To promote in Parliament better understanding of the UK’s armed forces, and their objectives and activities.” Organises receptions for troops returning from combat.
Income: £39,300
Supporters: BAE Systems (£8,700), Babcock International (£8,500). Lockheed Martin (£2,000)

Parliamentary Internet and Communications Technology Forum

Co-chairs: Stephen Mosley (Con), Chinyelu Onwurah (Lab)
Remit: Updates on digitial economy, to “provide a meeting place for Parliament, government, business and other interests to exchange information and opinions”. Runs IT campaign in schools.
Income: £209,728
Supporters: Motorola (£31,128), Vodafone (£6,000), Fujitsu (£6,000)