Have you over watered the basket? sounds like some sort of fungal attack?

Thank you for your help. I have just checked the soil and its quite wet. should I let it dry out completely?

What can I do if its a fungal attack?

What plants are you growing in your hanging baskets.
Although you need to feed and water hanging baskets regularly they also need good drainage.

Yes, that's vital so check for starters that water does run out. Let it dry out but not until its bone dry or things will start to wilt. Could you add a photo so we can see how bad it is? There are several anti fungal sprays you can buy and you could use one just in case, but it might just be the wet. Overwatering is as bad as underwatering as the roots need the tiny air spaces in the compost. You should always check that the compost is not wet already before adding more water.

Thank you for your help I will upload some photos so you can see the damage..