The Conservatives have confirmed that they are committed to a real terms increase in health spending over the coming decades.
Mr Lansley suggested other services would have to be cut
But senior party figures denied that meant they were now tied to massive unfunded spending commitments.
Shadow health secretary Andrew Lansley told the Times that health spending as a share of GDP would have to go up by 2% annually.
He later insisted this was not a pledge to increase spending.
He said it was a reflection of the realities of an ageing population - and it was "inevitable" that health spending as a share of GDP would go up from 9% to 11% by 2023 even if cost savings are made in the NHS.
But Health Secretary Alan Johnson accused the Conservatives of "confused rhetoric" over their spending plans, and said he suspected that Mr Lansley's comments may have been "a gaffe".
Mr Lansley told BBC News Radio 4's The World at One: "I wasn't talking about any projection for public spending and I wasn't making any commitment beyond that which we have made, which is up to 2011."
He said an increase in spending was inevitable with growing problems such as obesity and alcohol-related disease - and if productivity was not improved.
"Even if we do succeed (in improving efficiency) we should not assume that that means that we will not have a necessity for rising health expenditure - we will.
"And that is simply the product of an ageing population, rising technology and rising patient expectations."
The Conservatives have been committed to a real terms increase in NHS spending since 2006 - and have promised to stick to Labour's spending plans for the first three years of a Conservative government.
Beyond that, Mr Lansley suggested in his Times interview, the party could follow the projected growth in health spending recommended by former banker Sir Derek Wanless in his 2002 review of the future of the NHS commissioned by the then Chancellor Gordon Brown.
He told The Times: "I think we are bound to have rising real-terms health expenditure.
"That means that health expenditure is going to be a rising proportion of total public expenditure."
He also suggested other areas would have to be cut to pay for an increase in health spending.
"It's tough. It means there are places where public expenditure will decline as a proportion of GDP or in some cases in absolute terms."
He said it was "not for me" to say where the cuts would come.
In response, Mr Johnson told a pre-recorded interview for GMTV's The Sunday Programme: "I think that what Andrew Lansley was committing himself to was an extra £28 billion to be spent on health, taken from the budgets of other public sector departments."
"If it wasn't a gaffe, it still leaves us in a position where the Tories are desperate to say that they're going to retain the investment and indeed increase the investment; certain parts of their party are convinced there are going to be tax cuts; and they also have to share the proceeds of growth. So I think it's a pretty confusing situation."
Chief Secretary to the Treasury Yvette Cooper said Mr Lansley's comments showed the Conservatives had lost control of their tax and spending plans.
"Just hours after George Osborne was making more unfunded tax cut promises, Andrew Lansley has now pledged billions of pounds of extra health spending and neither of them have a clue about where the money would come from.
"It is clear from this ever growing list of huge, unfunded tax and spend policies that shadow chancellor George Osborne has completely lost control and that the Conservatives still aren't willing to face up to the tough decisions to keep Britain's economy strong and stable."
Mr Osborne has said the Conservatives long-term ambition is to cut overall public spending as a proportion of the national income.