The Apprentice star has featured in several campaigns for the Government-backed financial firm for five years but Cabinet Office rules ban the use of political appointments in government related advertising.
Sir Alan was appointed by Gordon Brown as enterprise champion in the Department for Business, Innovation and Skills last month as part of the Prime Minister’s reshuffle.
Despite his appointment, Sir Alan will continue to front BBC show The Apprentice. The latest series finished last month and was won by Yasmina Siadatan.
NS&I is an Executive Agency of the Chancellor of the Exchequer. It is one of the largest savings organisations in the UK, with almost 27 million customers and over £94 billion invested.
Earlier this year, it launched a marketing campaign featuring Sir Alan alongside Bob Geldof, Stephen Hawking and Germaine Greer.
Sugar has previously stated that any fee received from his appearance in NS&I campaigns has been donated to Great Ormond Street children’s hospital.