It's needed regardless of surname as taking baby abroad without written consent is considered child abduction.
https://www.gov.uk/permission-take-child-abroad It seems that a letter is enough with their contact details. I'd probably take the babies birth certificate and marriage certificate alongside that.
I have flown 5/6 times solo with my son from age 6months and we have a different surname. I’ve always taken his birth certificate and a dated signed letter simply saying I authorise travel to X on these dates signed by my husband with his address, mobile and email address. I have never been asked to show anything ever, just gone through as normal. Bette to take it though just in case.
@Alina, if your baby has British citizenship you can travel without consent. Get a signed letter from the father with his address and phone number and take baby’s birth certificate and you will be fine. I travelled with my baby last June to Romania for 3 months and when I came back they asked for the birth certificate additional to the passport as my baby has different surname than me, and I had no issues.
I have traveled solo several times a year with my son who is now 5 and now my baby. we don’t have the same surname. I always bring the birth certificates. I used to bring a letter to but was never get asked for that. I always get asked for their birth certificate when I come back into the uk. They quite often ask him who I am which is always interesting as he looks at them like they are crazy.
Thanks for your replies 🙂 In my country you need a letter from a solicitor stating that you are taking the child legally, so I wasn't sure about the process here in the UK
Do you need one if you are married? If you and baby have the same surname I don't see why you would?