Apple has revealed the NHS Covid-19 app was the second most downloaded iPhone product on its App Store in the UK this year.
Only the video chat app Zoom was installed more times. TikTok came third.
The contact-tracing app's achievement is notable given it only launched about two months ago and is solely targeted at users based in England and Wales.
The iPhone is the UK's best-selling smartphone brand.
However, more people use Android. The BBC has asked Google where the government's app appears on its own Play Store's rankings.
As of 18 November, the NHS Covid-19 app has been downloaded 20,056,685 times across both platforms, according to the government's latest figures.
The BBC has asked how many users have subsequently deleted it.
However, the Department of Health and Social Care has declined to provide the information at this time.
Researchers at Oxford University initially said that 80% of UK smartphone owners would need to use the app to halt the pandemic rather than just slow it.
But in a follow-up paper they produced evidence that the number of hospital admissions and deaths could be "meaningfully reduced" at a much lower level of uptake.
One issue that has deterred some people from using it has been that those on low incomes do not currently qualify for a support payment if the app tells them to self-isolate.
This contrasts with the £500 grant they can apply for if they are told to stay at home by human contact-tracers.
However, the BBC has learned that the app will soon generate a unique code that will make it possible to apply for a corresponding payment via an external website.
Top free iPhone apps in UK (2020):
- NHS Covid-19
Top paid iPhone apps in UK (2020):
- Driving Theory Test 4 in 1 Kit
- Official DVSA Theory Test Kit
- Procreate Pocket
- The Wonder Weeks
Top free iPhone games in UK (2020):
- Among Us!
- Brain test: Tricky Puzzles
- Subway Surfers
- Woodturning 3D
The pandemic also had an impact on the UK edition of TikTok's end-of-year review.
It said the biggest trend was #blindinglights - a dance challenge that became popular at the start of the first lockdown, featuring NHS workers and families moving to The Weeknd's Blinding Lights track.
This was followed by #isolationgames, a challenge organised in partnership with Team GB for users to perform sporting activities at home, to help make up for the postponement of the Tokyo Olympics.
Download numbers for the NHS Covid-19 app look good, and compare well with those for similar contact-tracing apps in other countries.
But we are still missing out on some key data which would tell us how well it is working.
We need to know how many people have deleted the app and - the key number - how many users have received notifications to go into isolation because they have been in close contact with someone who has tested positive for the virus.
The nature of the Google-Apple system underlying the app means only limited data is available to the government.
But there should be some information which would illustrate how effective the app has been over the last two months in its core mission of contact-tracing.
What we won't know is how many people have obeyed instructions to self-isolate.