Lobbying firms will be free to keep their clients secret – despite a new bill promising transparency in Whitehall – if they limit their meetings to special advisers and mid-ranking civil servants, it emerged on Wednesday.

Instead, lobbyists will only have to publish a list of their clients if they have had meetings with ministers or permanent secretaries who run Whitehall departments.

The new “lobbying register” will also exclude companies – such as those in financial public relations – for whom lobbying is only a small part of the business.

Those major loopholes in the imminent lobbying bill raised questions over whether the new legislation would force full client disclosure from Crosby Textor, Lynton Crosby’s lobbying firm which also advises the Conservative party.

David Cameron came under further pressure on Wednesday to clarify Mr Crosby’s links to the tobacco industry as Labour questioned why the government had dropped plans for plain cigarette packs.

Ed Miliband, Labour leader, wrote to the cabinet secretary asking him to investigate the “possibility of a conflict of interest”.

The prime minister said the decision to abandon the plans was made by him and health secretary Jeremy Hunt alone. He repeated his line that he had not been “lobbied” by Lynton Crosby over the issue.

Mr Crosby, a renowned election strategist, also runs a lobbying company called Crosby Textor. It does not disclose its clients but it has emerged that they include companies in tobacco, alcohol, oil, airports and property development.

The Labour party went on the offensive over potential conflicts of interest during prime minister’s questions, with leader Ed Miliband dubbing Mr Cameron the prime minister for “Benson and Hedge funds”.

The coalition said it had put the tobacco decision on hold until more evidence was forthcoming from Australia, where plain packs were introduced last December.

A spokesman for the prime minister refused to say whether Mr Cameron had ever checked who Mr Crosby worked for when he hired him last year.

The government’s lobbying bill is meant to force all third-party lobbyists to publish a full list of their clients.

The lobbying bill is controversial because it only applies to “third party” lobbying firms. It would not apply to “in-house” public affairs professionals at groups such as the CBI, Greenpeace, the RSPB or large companies.

Iain Anderson, deputy chair of the Association of Professional Political Consultants, said the plans would result in “less transparency” because fewer organisations and individuals would have to register than under the industry’s self-regulatory regime.

Mr Anderson said the APPC had carried out research into meetings between business ministers and lobbyists in 2012: out of 988 meetings only two were with “third party” advisers.

“The new bill will not capture the 99 per cent of lobbyists who meet regularly with ministers,” he said.

Copyright The Financial Times Limited 2018. All rights reserved.