Replace transparency in PNG image with white background

I have a PNG image with an alpha channel (i.e. transparency), and I need to create versions with the image layer composed onto a white background. I want to use a scriptable command using a CLI tool such as Image Magick to directly convert PNG to PNG losslessly.

An example of a non-working Image Magick command which results in an error is:

convert input.png -background white -flatten output.png

This works for me:

convert -flatten img1.png img1-white.png

Documentation references:

• -flatten command-line option
• -layers command-line option (-flatten is equivalent to -layers flatten)