Suspected cancer sufferers are avoiding hospitals because of the coronavirus, leading to fears cases may be missed.
Health Secretary Matt Hancock said the number of "first presentations" had come to a "juddering halt" - which was a big worry.
It comes after Richard Sullivan, a cancer professor at King's College London, warned the delays in cancer treatment would lead to more deaths than from coronavirus over the next five years.
Professor Sullivan said: "The cessation and delay of cancer care will cause considerable avoidable suffering. Cancer screening services have stopped, which means we will miss our chance to catch many cancers when they are treatable and curable, such as cervical, bowel and breast.
"When we do restart normal service delivery after the lockdown is lifted, the backlog of cases will be a huge challenge to the healthcare system."
Mr Hancock said that was "unlikely."
But he admitted there were concerns about the impact the virus might have on others needing treatment.
He told the Commons Health committee: "There is one area where I'm really concerned because we've also seen a drop-off for first presentations and this is a really big worry of mine.
"Which is that even though cancer treatment is ongoing now and if you need chemotherapy then that's ongoing, if you need radiotherapy that's ongoing."
He added: "But far, far fewer people are coming forward and this worries me because we'd spent an awful lot of time over several years, time when you (Committee Chair Jeremy Hunt ) were health secretary and time that I've been, driving up those coming forward for cancer treatment.
"And that has been brought to a juddering halt by this virus and we should all send a message to everybody who thinks they may have found a lump - phone your GP and you will get safe treatment even during this virus."
