Renewing lost passport, but don't know number

AprilKid

Hello everyone,
My sister has either lost or misplaced her passport, she doesn't know when/where it happened. 
The passport was many years out of date and she now needs a new one urgently for ID purposes. She is hoping to obtain it via the 1 week fast track service. 

The paper passport form (from the post office) asks for the lost/stolen passport number, date of issue etc. But she doesn't know this information.

What should she do?

Also, does she first have to report it lost?

If someone has experience of this, or understands it, please do let us know what the correct procedure is. 

Many thanks. 

onashoestring

Yes she should report it lost immediately. You can report it lost misplaced online . You can report it even you don’t know the number - but it may take them longer to cancel it .
https://www.loststolenpassport.service.gov.uk/

smudge56

All she needs to do is add how she lost her passport and when on section 3 where it asks what happened to the passport it will ask where is was lost and when - if she doesn't know just say the date she discovered it was lost and the last time she saw it.  She can also report it lost online but the L number is not required to submit the application.  If the Post office won't accept it - save  some money and just post it yourself

AprilKid

Thanks smudge56.