Mr Lewis will be expected to overhaul Downing Street's media operation
Prime Minister Gordon Brown has hired the public relations officer credited with boosting the Queen's image after the death of Diana, Princess of Wales.
Simon Lewis, who has also been employed by Vodafone and Centrica, will take over as the PM's official spokesman and director of communications.
Mr Lewis, whose brother Will is editor of The Daily Telegraph, will take charge of Mr Brown's media strategy.
He replaces the prime minister's outgoing spokesman, Michael Ellam.
Mr Lewis will, like his predecessor, be a civil servant and not a party spokesman, the BBC's political correspondent Nick Robinson said.
He added that Mr Lewis's main task will be "to repair the dreadful damage done to his boss's reputation, in part by the actions of brother Will over at the Telegraph".
The Daily Telegraph has been publishing leaked details of MP expenses claims over the past few weeks - details which were partly blamed for Labour's disastrous showing in the English local and European elections earlier this month.
In 1998, Mr Lewis joined Buckingham Palace as communications secretary on a two-year secondment from energy giant Centrica, and was given the job of creating a new PR strategy for the Royal Family in the wake of Princess Diana's death.
He was director of communications at the SDP, which later merged with the Liberal Party to form the Liberal Democrats, and is reported to be on good terms with Business Secretary Lord Mandelson.
"Simon Lewis is a very experienced public relations professional with a very strong track record working in both the public and private sectors," Mr Brown's current spokesman said.
Mr Ellam returns to the Treasury, where he was previously a senior civil servant, at the end of July.