It is a FREE NOW policy and the law that you must always allow service dogs to accompany their passengers for a trip. Refusing to do so is a serious offence that risks prosecution and the possible loss of licence.
People use service dogs to help them with a range of disabilities, such as deafness or blindness, however, not all disabilities are immediately apparent.
What if the animal isn't a service animal?
With non-service animals, your vehicle is your property and you have the right to refuse entry to riders who are accompanied by non-service animals.
What if I am allergic to dogs/animals?
If for medical reasons you're not able to accept a service dog inside your vehicle then you must obtain an exemption certificate in advance from your local licensing authority, and have this in your vehicle for every trip.
Please check out TfL's website for more information.