Missing sign in button for copilot.microsoft.com

Starting today, the sign in button for copilot.microsoft.com is missing. It was fine yesterday, but when I turned my computer back on today, no sign in button.

I have tested in the following browsers on both Windows 11 and Ubuntu 25.04:

Edge

Firefox

Zen

In each case I have cleared the browser cache and cookies. I have rebooted each machine multiple times, no success.

The only way I can get into Copilot is via the Windows 11 app. I prefer working in Ubuntu because it's faster for my workflow.

Any guidance about what to try next will be appreciated.