This deep cut is the result of driving over a glass (not a piece of glass, but an actual glass. It was left on the street by a drunken? reveller. It was a gin-and-tonic glass, so at least it must have been a stylish reveller). The inner tyre received a similar cut.

Replacing both tyres is the only possibility. I didn't mind replacing the inner tyre, because it was bad and needed replacing. I replaced the outer tyre not so long ago, so that was not funny. As this is the rear wheel and I have a three-speed gearbox by Sturmey Archer, I need a special tool in order to prevent complete dismantling of the wheel (cumbersome).