You’re right to be concerned about the message that you give to prospective employers about why you want to leave/have left your existing job but there are a number of ways that you can get round this.
First of all, as Loztiggy suggests, you could state that the job was not utilising your skills and that there was no clear career path for you. In that way, you are turning a negative into an impression that you have ambitions and want to fill a more productive role.
Alternatively, you could say that you felt that you were following the wrong career path and that you felt that you had to take positive action to correct this. Just be careful that you don’t end up with a period of unfilled time on your CV (try volunteering?) while you look for a different job.
It can be OK to simply say that you felt unsuited to your previous job but you should always balance this by stating that you had been keen to take on more responsibility or that your skills are more aligned with the job that you are applying for.
It’s always a great approach to turn the issue around and tell the prospective employer that you have applied to them because they have an excellent reputation and you see them as a leader in their field. If you add something like this at the end of your explanation for leaving your previous job, they will remember more of the positives you have mentioned about them rather than why you left!
Good luck.