US passport or UK passport

Just read something in another thread that made me wonder something about passports? I have an expired US passport and a new UK passport that was a first issue, as I just received my UK citizenship. I did not intend on renewing my US passport when we leave for the UK, as I thought there was no need for it. Do I need my US passport to leave the US or can I use the UK passport?

Just read something in another thread that made me wonder something about passports? I have an expired US passport and a new UK passport that was a first issue, as I just received my UK citizenship. I did not intend on renewing my US passport when we leave for the UK, as I thought there was no need for it. Do I need my US passport to leave the US or can I use the UK passport?

You need your US passport to leave the US. 

Yep echoing what expat has said you  need a valid US passport to enter and leave the US and the valid UK one to enter and leave the UK if you have the two passports thats how it works.. 
Hope you aren't traveling to the US too soon.. .