He also demanded ministers at least guarantee there will be no relaxation of mixed-sex ward rules, a practice that the government had promised to eliminate because many patients find it humiliating and inappropriate.
His concerns have also been raised by experts, who said a recent government pledge to invest £145m in emergency department upgrades and 900 more beds will “not scratch the surface” of the shortages gripping the health service.
Speaking to The Independent on the eve of Labour’s annual conference, the shadow health secretary said patients should be concerned about a repeat of “terrible scenes” where sick and elderly people were left languishing on hospital trolleys.
He said: “I’m very worried and I think the government has shown a breathtaking complacency so far. We had quite a devastating winter on the back of another very serious winter.
“The previous winter was described as a humanitarian crisis by the Red Cross, for goodness sake.
“This winter, we saw blanket cancellations of non-urgent operations and again we saw terrible scenes on the televisions of people, often elderly, confused, people languishing on trolleys for hours in corridors.
“I’m really worried that we are on the brink of another serious winter of cancellations, of people trapped in ambulances in the cold, confused, in the dark, unable even to get into hospital.”
He added: “Patients should be concerned. Nobody wants to see their grandmother lying in a state of confusion, of distress on a trolley for hours and hours and hours in a corridor in a hospital.”
Both Theresa May and ex-health secretary Jeremy Hunt were forced to apologise in January when hospitals were told to defer around 55,000 operations to free up capacity for the sickest patients.
Mr Hunt even acknowledged that it was the “worst ever” for the NHS, compounded by flu outbreaks and shortages of beds.
Doctors have also warned that the NHS is facing a strain on services in the summer months which is usually seen in the winter, when poor weather and seasonal flu heap pressure on hospitals and GPs.
“We’ve got no clarity on whether elective operations will be cancelled again,” Mr Ashworth said.
“I suppose the challenge for the new health secretary is to tell us whether he will be imposing a blanket cancellation of operations like his predecessors did this year.
“The other challenges for the health secretary is to tell us whether mixed sex breaches will go up like they have done.
“Of course we are supposed be stamping out mixed-sex ward breaches because we know it can be undignified for the patients.”
If elected, Labour would set out plans for a £500m winter bailout fund to allow local commissioning groups and hospital trusts to decide where the need is greatest, such as social care, staffing or hospital capacity.
The government announced £145m would be given to emergency departments from its existing budgets towards more beds earlier this month.
To mark the NHS’s 70th birthday, Ms May pledged an additional £20bn a year for the health service by 2023. However this funding doesn’t begin until next year.
Mr Ashworth also set out his concerns about Brexit, saying its potential impact “hangs like a sword of Damocles” over the health service. He also branded the government’s no-deal planning “useless”.
The Leicester South MP said: “I will be insisting in any deal, that the NHS is safeguarded, not just the staff, which gets well talked about, but our access to medicines, the way in which medicines cross the border on a daily basis but also our ongoing access to various research initiatives and research fields.”
He said Labour have to “wait and see” what deal the prime minister brings back from Brussels before it could decide whether to vote against it.
Asked if he would back a second referendum on the final deal, he said: “Our focus is the vote in parliament but if parliament can’t agree then Keir Starmer [shadow Brexit secretary] and John McDonnell [the shadow chancellor] have said we won’t take anything off the table.”
Brexit divisions could dominate Labour’s conference in Liverpool this week, as local constituency parties have put forward around 150 motions for discussion on Britain’s exit from the EU.
Mr Ashworth said: “We are a pro-European, internationalist party and we are very proud of being a pro-European internationalist party, and it is something which I think the delegates at conference will feel very passionate about.
“They will want to, I’m sure, debate the issue properly and thoroughly.
“But I think people understand our stance, they understand that we want a good deal for the country but I think our delegates will share our scepticism that Theresa May is equipped to deliver this deal.”
However he claimed the party is “more united on this issue than sometimes people on Twitter suggest”.