the pandemic has worsened "non-Covid" health problems.
He said many people had chosen not to go to hospital for fear of catching the virus, but also blamed a "very British" attitude of not wanting to heap too much pressure on the NHS.
England's NHS waiting list last month reached the "grim milestone" of five million for the first time since records began.
Over a quarter of a million Brits have waited more than a year to start hospital treatment, with appointments pushed back by the pandemic.
Around 385,490 had to wait more than a year, with 5.12million still on the waiting list at the end of April.
Experts warned that delays on this scale can "cost lives", and the British Medical Association said the backlog is "dangerously close to crippling" the health service.
Mr Javid now fears the number of patients waiting for treatment could more than double after being shown "shocking" modelling data.
The NHS faces a huge backlog due to the pandemic Credit: Getty He said: "What shocked me the most is when I was told that the waiting list is going to get a lot worse before it gets better.
"It’s gone up from 3.5 million to 5.3 million as of today, and I said to the officials: 'so what do you mean [by] a lot worse', thinking maybe it goes from 5.3 to six million, seven million.
"They said no, it's going to go up by millions... it could go as high as 13 million.
"Hearing that figure of 13 million, it has absolutely totally focused my mind, and it’s going to be one of my top priorities to deal with because we can't have that."
Mr Javid has vowed to get "creative" to help clear the logjam "as quickly as possible" as one of his "top priorities".
This will likely include increasing the number of virtual appointments available - particularly for younger people - and turning to private hospitals to provide care funded by the NHS.
"If paying private hospitals to carry on treating NHS patients can help to clear the backlog and ensure people can be seen a lot more quickly by the NHS paying for them to go to an independent provider, that's fine with me," he said.
But Mr Javid worries that even with these extra measures, the ever-growing waiting list "will take some considerable time to clear".
Hearing that figure of 13 million, it has absolutely totally focused my mind, and it’s going to be one of my top priorities to deal with because we can't have that.
Sajid Javid Health Secretary Last month The Sun revealed that hospitals were being told to brace for up to 12.2 million people in need of elective procedures which include hip, knee and eye operations.
And for the first time since records began, patients are waiting more than two years for hospital treatment.
Research suggests a further 2.4 million operations will have been cancelled by the end of 2021.
Around three million people have missed out on cancer screenings during the pandemic , including Corrie star Kym Marsh’s beloved dad who’s fighting for his life after undiagnosed prostate cancer spread to his pelvis, spine, ribs and legs.
The emotional star filmed a video for the BBC with her terminally ill dad and explained: “Dad knew something wasn’t right but he felt he couldn’t go to the GP during that time [the coronavirus pandemic] so it got left.”
Pat Price from Imperial College London and co-founder of CatchUpWithCancer added: "The delays to cancer patients are an unmitigated disaster.
"Thousands of cancer patients are likely to die unnecessarily.
"It doesn’t have to be this way - we need serious investment and a proper national plan, now."
UK Covid hospital admissions surge by more than HALF in a week with 563 patients in 24 hours