Labour leadership contender Owen Smith will accuse the government of being behind “a secret plan to privatise the NHS” in England.
In a speech later, Mr Smith will say NHS spending on the private sector has doubled from £4bn to £8.7bn since the Conservatives returned to government six years ago.
The Department of Health said his analysis was “simply wrong”.
Mr Smith is campaigning to replace current Labour leader Jeremy Corbyn.
He will point out that the recently published Department of Health annual report showed that almost 8% of healthcare in England last year was bought from private sector providers – up from 4% in 2010.
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Ahead of his speech on the campaign trail in Salford, Mr Smith said: “The NHS is our country’s most valued institution – and people will be shocked to hear that the Tories have been putting together a secret plan to privatise it.
“We all rightly contribute to the NHS through our taxes – but we must make sure that money is spent on doctors and nurses, and not lining the pockets of private sector shareholders.”
A spokeswoman for the Department of Health said: “This analysis is simply wrong – this government was the first to ensure that doctors, not politicians, make decisions about who provides care.”
She added that the rate of growth in the use of the private sector was slower than when Labour was in power.
“We are committed to the values of the NHS and to delivering a safer seven day service. We are investing an extra £10 billion a year by 2020 so the NHS can introduce its own plan for the future,” she said.
Chris Mason, BBC political correspondent, said in this keenly fought Labour leadership contest “being seen to be socialist is essential”.
“Owen Smith will hope this speech, aimed at the left wing electorate of Labour members and supporters who will chose their party’s leader, will help burnish his claim to be both radical and effective at holding the government to account,” he said.
Last month Mr Smith’s rival, Jeremy Corbyn, took a dig at his rival for comments he made about “choice” and private health providers when he worked as a lobbyist for drug company Pfizer in 2005. Mr Smith told the BBC he believed in a “100% publicly-owned NHS free at the point of use”.
Meanwhile, Mr Corbyn has been focusing on education.
In an interview with The Observer he said he wanted a “National Education Service” based on the principles of the NHS meaning free education for all, “from cradle to grave”.
The Labour party has been embroiled in bitter in-fighting since Mr Corbyn’s leadership was challenged.
Rows escalated over whether the massive influx of new members that have signed up to the party in recent months should be allowed to vote in the election, leading to claims that hard-left activists were trying to infiltrate and sway the result.
Labour’s National Executive Committee (NEC), the body that governs the party, ruled that full members could only vote if they had at least six months’ continuous membership by 12 July.
The party offered a window from 18-20 July when more recent members – and non-members – could pay £25 to become “registered supporters” and gain the right to vote.
On Friday the Court of Appeal ruled Labour was within its rights to stop some 130,000 new members who did not meet this criteria from voting.
The exclusion of the new members is thought to benefit Mr Smith.
Mr Corbyn has made a dig at his rival’s former job with Pfizer, who he was a lobbyist for in 2005,