Deputy Prime Minister Nick Clegg described Mr Johnson's move into a ministerial role as "a political appointment for the Conservative Party".
Mr Clegg told LBC 97.3 radio:
This is a political appointment for the Conservative Party, if you like, about how the Conservative Party generates policy ideas, and I don't think you can say the Prime Minister as leader of the Conservative Party is not entitled to appoint a Conservative MP to come up with policy ideas for his party.
Boris and Jo Johnson share the family's trademark ultra competitiveness, but in many ways they are very different people.
Jo Johnson will head up the Policy Unit inside Downing Street in a shake-up to inject fresh Tory thinking into Government.