UKTI is the Government's international business development organisation, which brings together the work of its two parent departments: the Foreign and Commonwealth Office (FCO) and the Department for Business, Innovation and Skills (BIS).
Lewis was immediately replaced as Downing Street director of comms by Andy Coulson, until yesterday the Conservatives' comms director, and a former editor of News of the World.
However Prime Minister David Cameron's official spokesman is understood to be Steve Field. He is currently director of communications at HM Treasury. Previously he was head of the Treasury's European Economic Reform Team.
In Lewis' new role, which he will take up with immediate effect, he will report to the chief executive Sir Andrew Cahn.
Initially he will review the UKTI's digital communications strategy, advise on the role and responsibilities of its communications function and, working with the FCO, look at how the UK's international business reputation can be enhanced.
Lewis said: 'It has been an enormous privilege to work in No10 over the last year and to have served as the Prime Minister's official spokesman. I have also very much enjoyed the managerial challenge of running the wider communications activities in No10.'
Lewis became the Prime Minister's spokesman last summer. He came to national prominence when he began working for the Queen in 1998, helping stabilise Buckingham Palace’s reputation after the death of Diana, Princess of Wales.
Cabinet secretary Sir Gus O'Donnell said of Lewis: 'I would like to thank Simon for all he has done in one of the most difficult and challenging jobs in Government over the last year, and for his contribution as a senior civil servant in No10.'