I know it's a back wall, but its' removal will still potentially compromise overall stability of the house - see John Ds comment above.
One way to avoid this is to leave reasonable returns at each end; alternatively have a goal-post frame made. There are other ways of proving compliance, depending on the layout of the house.
When the design is put to Building Control, this issue will be raised, so you and your SE need to be aware of the design/cost implications.
