Is a letter from the sponsor required or optional? I have not seen any formal statement that such a letter is required.
The letter from the sponsor is required. It should outline how you meet each requirement for the visa:
- ability to sponsor the visa (UK citizenship)
- how you are meeting the financial requirement and under which category
- where you will live together in the UK
- a short history of your relationship (1 paragraph)
The supporting documents guidance says the following:
Evidence of your relationship to your sponsor and any contact between you
This could be a letter from your sponsor confirming your relationship and that they are supporting you along with copies of:
- birth certificate or adoption certificate
- marriage certificate or civil partnership certificate
- death or divorce certificate
- photographs of your wedding, civil partnership ceremony or other time spent together
- phone records
- emails, letters or cards
* You should not submit DVDs or video cassettes*While is does say 'could be', it's really a 'you need to' - we have seen visas refused in the past because a sponsor letter has not been included.