Labour's shadow chancellor John McDonnell has said the party's 32-hour week policy would "apply to everybody" - including NHS staff.
The party wants to introduce the plan - the equivalent of a four-day week - over a 10-year period.
Earlier, the shadow health secretary said claims the NHS would switch to a four-day week were "nonsense".
But Mr McDonnell said Jonathan Ashworth was just referring to a suggestion that it would be "an overnight thing".
The shadow chancellor was speaking at an event to set out plans to spend more on the NHS in England than the Conservative Party if Labour wins the general election on 12 December.
Health Secretary Matt Hancock said Labour's "chaotic" plan for a four-day week "is a cost the NHS can't afford", saying it would "cripple our economy and cost the NHS billions every year".
When asked on BBC Breakfast whether the NHS was part of Labour's 32-hour working week policy, Mr Ashworth said: "No."
"John McDonnell wants to set up a commission to see if there are ways in which, in 10 years' time, people's working hours can be reduced," he said.
"The idea that the NHS is going to go to a four-day week on 13 December if Labour get in is just for the birds - it's nonsense."
But when shadow chancellor John McDonnell, who first unveiled the plan at Labour's party conference, was asked to clarify if this was indeed the case, he said: "It's a 32-hour week implemented over a 10-year period, it will apply to everybody.
"What Jon was talking about this morning was how you apply it over that 10-year period."
He said the Conservative Party had given the impression that it would be introduced if and as soon as Labour was elected on 12 December.
But, he said, it is a policy that has to planned and negotiated over time.
"It's not an overnight thing it's a realistic ambition," Mr McDonnell said.
When Mr Ashworth was asked again on BBC Politics Live to clarify his comments, he said: "The idea there is going to be a mandatory four-day week imposed on the NHS is not true."
He added that it will form part of negotiations with staff.
"When we go into negotiations with low paid staff in the NHS about future pay rounds, through the bargaining process, these are the things that will be looked at," he said.
"But people are only going to be able to reduce their hours in the working week after we've had a 10 year process of looking at it, of growing the economy and seeing what's possible."
Health Secretary Mr Hancock said: "Corbyn's health spokesman toured the TV studios this morning claiming that their plans for a four-day working week would not apply to the NHS.
"But now the shadow chancellor says it will after all. Voters deserve to know the truth."
He accused Labour of being "in complete chaos" and said a Labour government "will mean that patients will get worse care".