One of David Cameron's independent efficiency experts who identified the £12bn spending savings an incoming Conservative government could make this year chairs a private healthcare firm that openly admits it will benefit from NHS spending cutbacks.

Sir Peter Gershon chairs General Healthcare Group, the largest private sector health firm in the UK. The Conservatives have relied on Gershon's analysis of efficiency savings to enable them to promise scrapping most of the government's planned national insurance increase – a move that has left Labour flatfooted at the outset of the election campaign.

The disclosure, which will open the Tories to the charge that they have not been transparent about the interests of a key adviser, came after the issue dominated the second day of formal campaigning. Cameron pummelled Gordon Brown over Labour's insistence that it had to raise NI contributions (NICs) – rather than cut spending immediately – in the last prime minister's questions before polling day, and the Tories announced that they had secured the support of another 30 business leaders, taking to 68 the number who have backed their plans to scrap the rise.

The Conservatives claim that the £12bn savings would enable them to cut spending this year by £6bn and channel a further £6bn into other areas. But at a potentially crucial press conference tomorrow – at which Brown and Alistair Darling, the chancellor, will attempt to stem the damage caused by the business assault on the NI rise – Labour will argue that it would be more damaging to take £6bn out of the economy this year than to increase Nics next year.

They will also seek to rebut the Tory claim that Labour is not willing to make efficiency savings of its own in the current financial year.

Senior Labour figures privately acknowledge that they are being severely damaged by the NICs issue, although they will draw some encouragement from a Times Populus poll that shows a Tory lead of seven points, short of the lead needed to give the Conservatives a majority.

General Healthcare Group, the company chaired by Gershon, predicts in a strategy paper published on its website that the NHS may face a "very severe contraction in its finance with an £8bn-£10bn cut in real terms likely in the three years from 2011".

It continues: "Given this lack of funding growth, there will be an increasing role for the private sector, even if NHS efficiencies can offset some of the budget pressure."

The company has also just commissioned a report from the Economist Intelligence Unit that calls for "new thinking on funding and a growing role for the private sector".

In his paper for the Conservative party identifying £12bn efficiencies this year, Gershon suggests: "A new government needs to give the best providers real control over the process of delivering a service, holding them to account for the end outcome, not the details of how the service is delivered."

He lists private firms as one of the future providers of services designed to achieve a radical re-engineering of public services where quality is improved while costs are substantially reduced.

Gershon is a recognised expert in improving government efficiency and there is no suggestion that his report to the Tories was influenced by the fact that his firm stood to benefit from spending cuts. Labour also supports the use of private firms in the NHS to extend choice.

A Tory source said the charge of lack of transparency was "nonsense", adding: "Not only did Gordon Brown appoint Sir Peter Gershon as an efficiency adviser twice but, under our plans, the Department of Health is protected – efficiency savings will be invested back into the frontline." But David Cameron made no mention of Gershon's business connections when he presented the efficiencies report from Gershon and Dr Martin Read.

Gershon did not respond to a request for comment tonight.

The Conservatives have built their campaign to rein back NI contributions increases next April largely on the two men's advice that the government can make an additional £12bn in efficiency savings this year over and above that planned by Labour.

Labour will release a 15-page paper attacking the Gershon and Read proposals, and will also point to a prediction from the Organisation for Economic Co-operation and Development today that Britain will experience annualised growth of 2% during the first three months of 2010, ahead of Germany, Japan and Italy. Labour claimed that the OECD's chief economist, Pier Carlo Padoan, had endorsed the Brown position. "Although we are seeing some encouraging signs of stronger activity, the fragility of the recovery, a frail labour market and possible headwinds coming from financial markets underscore the need for caution in the removal of policy support," he said.

Brown found himself on the ropes over NI today at what was possibly his last prime minister's question time. "If they know there is waste … why not cut out the waste now to stop putting up the taxes later?" Cameron said.

"You should not have to pay more taxes to pay for government waste. Some people say: where are you going to find this money? What we are talking about is saving one pound out of every hundred pounds the government spends. I do not think saving one pound in every hundred is particularly tough."

The Tories later accused Brown of being at war with business after the prime minister, in an interview on GMTV, joined the business secretary, Lord Mandelson, in saying leading businessmen were being deceived by Tory propaganda. He later tried to avoid repeating the charge.

The latest business people to endorse the Tory plan to halt the planned NI contributions rise include the Corus chief executive, Kirby Adams, the Northern Foods chief executive, Stefan Barden, Reed Elsevier's chairman, Anthony Habgood, and Jewson's chief executive, Peter Hind.

Read and Gershon each produced two sides of A4 for the Tories last week to claim how the £12bn in efficiencies could be made in 2010-11 in addition to those already earmarked by the government. They produced a range of very broad measures including recruitment freezes, a one-year freeze on all new IT projects, renegotiating contracts and selling government property.

Read has held the chairmanship of the remuneration committee at British Airways since 1990. During that time, the basic salary of the chief executive, initially Rod Eddington, has risen from £486,000 to £735,000 in the 2008/2009 financial year (when Willie Walsh is chief executive).