Upvote:0
To add to jpatokal's answer, the best thing you can do is contact a UK embassy or consulate, explain your situation, and ask for your options.
Maybe it's possible to get a temporary waiver added to your UK passport, or get an expedited or temporary US passport issued by them for the duration of the trip.
Check your options, I'm sure it's hardly the first time the people at the embassies and consulates have had to deal with situations like yours and they're bound to have procedures for it.
Upvote:5
It would be illegal for you to enter the US on a non-US passport. US Embassy in Sweden:
According to Section 215 of the U.S. Immigration and Nationality Act (8 U.S.C. 1185), it is illegal for a U.S. citizen to enter or leave the U.S. on anything other than an U.S. passport. This applies to dual citizens as well, meaning that persons holding e.g. both Swedish and American citizenships and passports must enter and leave the U.S. on a U.S. passport. They may NOT enter/leave the U.S. on a Swedish passport. This applies to children as well as adults. There is no guarantee of entry to the United States without a valid U.S. passport.
Now, can you get away with it in practice? Possibly, only CBP knows what will "show on the screen" and they're not telling. But since they clearly already know your UK passport number and that you've applied for citizenship, it's a pretty big risk. You will also have to apply for an ESTA for your UK passport, which gives them another chance to catch you. All in all, I wouldn't risk it.