The Prime Minister was challenged over the claim on Tuesday, and said the pre-pandemic backlog had been expected to improve before widespread industrial action was launched.
It comes amid the longest joint strike by junior doctors and consultants in England, coinciding with the Tory Party conference in Manchester.
He told ITV’s Good Morning Britain: “The reason that waiting lists are going up is because we’ve got industrial action because doctors are on strike. Now, I’ve sat down and tried to do the reasonable thing…”
GMB presenter Susanna Reid intervened to say: “No, that might be one of the reasons.”
“That is the number one reason. But if I could just finish, if I could just finish, it’s really important…”
Mr Sunak was told there was “already a backlog” preceding the strikes, before replying: “We had stabilised it, it had stopped going up and it was forecast to start going down until industrial action started. That’s the reality of it.”
Consultants in England from the British Medical Association (BMA) have written to Mr Sunak saying they will not call any more strikes for four weeks to “facilitate negotiations taking place”.
Dr Vishal Sharma, chairman of the BMA consultants committee, has invited the Prime Minister to enter talks with conciliation service Acas “if necessary” to prevent walkouts running into winter.
Asked whether ministers are willing to meet medics, Mr Sunak told BBC Breakfast: “Of course we are, we’ve tried multiple times.”
But he defended the pay rise of around 9% already given to junior doctors, while the BMA has argued they need a 35% increase to make up for what it says are years of below-inflation rises.
Mr Sunak has made cutting the NHS backlog one of the five key priorities for his premiership but the numbers are going in the opposite direction.
Latest figures show waiting lists for elective treatment across England have risen from 2.6 million in 2010 to almost 7.7 million.
Health experts have acknowledged the impact of the strikes on the backlog but also cited staff vacancies and increased demand as contributing factors.
The BMA has said medics would provide emergency cover only during the three-day walkout – also known as Christmas Day cover – which began on Monday at 7am.
The letter states that if medics do not receive a “credible deal we can put to our members” by November 3, then strike dates will be set for November and December.
It comes as radiographers are set to join doctors on picket lines at hospitals across England, heaping additional pressure on NHS services already disrupted by strike action.
The Society of Radiographers (SoR) said staff at 37 trusts in England are set to strike for 24 hours from 8am.
Hospital dentists from the British Dental Association are also staging a three-day strike, providing emergency care only.
Doctors and radiographers are planning to travel from around the country to join a rally outside the Conservative conference, with some travelling from as far away as Bristol or Newcastle.
“They have suggested they won’t talk to us while strikes are called,” Dr Sharma will tell the rally.
“This is a ridiculous position for the Government to take. They should be doing everything, right up until the last minute, to avert strike action. Our door has always been open, and that remains the case.
“To demonstrate our commitment to resolving this dispute, yesterday I wrote to the Prime Minister.
“We have not yet called further strikes beyond this week, and have given the Government four weeks from today to enter formal talks and present us with a credible offer.
“We have also indicated that we are willing to involve Acas to conciliate a resolution.
“The ball is well and truly in the Prime Minister’s court. The Government has run out of excuses not to negotiate in good faith. But they need to know that, if they fail to negotiate, we are not going anywhere.”
Earlier this week, NHS England warned that there would be “extreme” disruption during the unprecedented strike.
It said routine care would be brought to “a near standstill” but urged the public still to come forward to seek emergency care when needed.
Meanwhile, health commentators have urged the Government and the unions to end the deadlock.
On Monday, Marina Glasgow, Acas’s chief conciliator, said the service has a team of experts who are ready to help with the disputes.
Sir Julian Hartley, chief executive of NHS Providers, described the letter as an “olive branch”, which trust leaders hope could be the first step to a resolution.
The NHS Confederation warned that unless the dispute ends, the health service will not achieve the Prime Minister’s ambition to reduce the NHS waiting list.
But the NHS Confederation is facing its own industrial dispute with a small number of staff taking strike action across sites in London and Leeds on Thursday.
Unite general secretary Sharon Graham said: “Our members at the NHS Confederation do vital roles and are asking that pay doesn’t go backwards in the midst of a cost-of-living crisis.”
Matthew Taylor, chief executive of the NHS Confederation, said: “We regret that a minority of staff within our organisation have taken the decision to strike on Thursday. Of the over 260 staff employed here, 22 voted in favour of this action following our recent pay review which saw an average pay rise of 5%.
“When we were notified of the concerns from Unite, particularly in response to the fact that 38 members of staff unfortunately would see a reduction in their take-home pay due to the ending of a non-consolidated payment last year, a one-off payment for all staff was offered in response. However, this was rejected by the union.”