Thanks for your responses. I will consider them closely. I changed my mind for a number of reasons:

I lack a passion for the project and planned to treat the PhD like a job
I have no desire to become an acedemic
I was strongly motivated to take a PhD to get rich, though I am now aware there are easier ways to do this
I want to move on and switch up cities, as I have been in one place since my undergrad and I am excitied for a change
I don't think I'm capable of completing the work required to get a PhD

Are these good reasons? I am open to suggestions please help me :)

Hi, I've been offered a PhD in a science field on the UK. I have funding and have signed up to start later this month. The issue is, I have decided I no longer want to do a PhD. I feel slightly trapped as my supervisor acquired funding for me. Additionally she is my supervisor for my masters project which I am handing in on Monday. What should I do? Is starting a PhD and quitting in the first few months a very bad decision? Thanks