You can not kill mold and mildew with soap or shower gel. Mildew spores are on of the most persistent and durable organisms on the planet.
Only certain types of bleach will kill them. It's a very common problem with boat sails as they are often put away wet and there is not much that can be done and that's on tough, heavily coated sail cloth!
Unless the tent is of exceptional quality and would retail for over £300, it's not worth trying. Then be prepared to ruin it. But don't waste you time washing it in detergent or soap at any temperature. Dettol do a spray called "Mold and Mildew Killer' available from some supermarkets or hardware shops. It's nasty stuff and will strip the colour from cloth (amongst other things!), but it does kill mildew.
Don't just try ignoring it either. The spores will spread and ruin everything else you own. They have also been known to cause respiratory problems in some people. Zipping yourself into a mildew infested tent doesn't seem worth the risk.
So to summarise:
1. If it's a cheap or reasonably priced tent: Throw it away immediately.
2. If it's a very expensive tent: Try killing the mold with Dettol spray, then wash, reproof and test it before finally throwing it away.