The shadow chancellor confirmed that a four-day working week would be part of Labour’s election offer and the radical proposals would extend to all workers.
However his comments set him at odds with Jonathan Ashworth, the shadow health secretary, who said Labour was not "imposing" a four-day week on the NHS in a round of broadcast interviews.
The splits emerged as the Tories launched a fierce attack on Labour's plans to overhaul working hours, claiming the cost of implementing the plans would wipe out the promised funding boost.
Mr McDonnell appeared to contradict his shadow cabinet colleague as the pair launched Labour's £26bn NHS "rescue plan" at an event in London.
Pressed on whether NHS staff could see a reduction in working hours, Mr McDonnell said: "It will apply to everybody - what Jon was talking about this morning was how you apply it over that 10 year period.
"The Tories have given this impression that we get elected on 12 December - I am grateful for their confidence in us - and on 13 December all of sudden everyone is on a four-day week.
"No, that's what we said. That's not what Jon said this morning either."
Mr McDonnell argued the move would allow all workers to share the advantages of rising productivity, which Labour believes will be triggered by the investment it plans for the economy.
Standing alongside Mr Ashworth, the shadow chancellor said: "It applies to everybody because as you grow the economy, employers and trade unions sitting down will be able to judge how that productive economy has grown, what the timing will be for the staggered reduction of workers hours, with that ambition in a decade.
"I don't think it's that ambitious. Over a decade, reducing to 32 hours a week is what realistically is what people will want and people will be able to afford."
Only hours before, Mr Ashworth appeared to water down the pledge by saying it would not be imposed on the NHS.
He told TalkRadio: "It's nonsense. He's [Matt Hancock] trying to make out that if I am elected on 13 December, I will be like, 'Right, you are all out, you aren't working on Friday.'"
"We are not imposing a four-day week on the NHS," Mr Ashworth went on.
"What John McDonnell has said is that over the next 10 years, let's look at the ways in which we can move to an economy where people are working less hours."
“If we can drive up productivity maybe we can get into a position where people can work less hours. He is going to set up a commission to explore that as a possibility.”
It comes as Labour unveiled a plan for a 4.3 per cent annual funding increase for health spending to boost care, address staff shortages and rebuild crumbling facilities.
Mr Ashworth warned that a decade of cuts has plunged the NHS into “year round crisis” and promise to increase the health budget to £178bn by 2023-24 – a real terms increase of £26bn – funded by plans to reverse corporation tax cuts and hike up taxes for the wealthiest in society.
The shadow health secretary also set hares running by refusing to rule out allowing freedom of movement to continue for EU citizens, who account for a significant chunk of the NHS workforce.
He said: "If a trust believes a nurse or a surgeon is qualified enough to come to this country to care for our sick or elderly, they should be allowed to come to this country."
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