how can I set htmleditorextender to readonly?

I have a textbox "txtMessage" extended by a htmleditorextender.

In a workflow, the textbox has to be readonly at a certain state.

txtMessage.Readonly = true or txtMessage.enabled = false has no effect.

htmlEdtitorExtender.enabled = false simply deactivates the extender.

If I disable the extender and set the textbox to reaonly, the textbox is reaonly but I'm loosing the formatting and that's not a real alternative anyway. Is there a simple trick (preferred from code behind) to set the corresponding textarea readonly?