Prime Minister Gordon Brown has brought in the chief executive of one of the city's best known communication firms to be his most senior special adviser.
Stephen Carter is a former chief executive of Ofcom.
Stephen Carter, group chief executive of Brunswick Group LLP, will be chief of strategy and principal adviser.
Mr Brown praise the "track record" of Mr Carter, 43, who is also an ex-chief executive of media watchdog Ofcom.
But the Conservatives say his role was to put "gloss on the failings of" Mr Brown's government.
In his new role, Mr Carter will co-ordinate policy, research and communications at No 10.
Mr Brown said: "I am delighted that Stephen Carter has accepted my invitation to join the Downing Street team as we build on our programme of reform and pursue an ambitious legislative programme.
"As both a chief executive and industry regulator, he has an enviable track record in the private and public sectors and will bring his considerable qualities and experience to the work of government."
The appointment will be seen as an attempt to beef up Mr Brown's inner circle in the wake of a difficult few months marked by crises and negative media coverage.
Those difficulties, on issues such as Northern Rock and the loss of 25m people's child benefit records, came after Mr Brown's last-minute decision not to hold a snap election in the autumn.
The prime minister, who was said by opponents to have "bottled it" in the face of poor polls, has since acknowledged that he should have put an end to the speculation sooner.
And Caroline Spelman, chairman of the Conservative Party, said: "Gordon Brown has provided no fresh start and no big vision.
"He should be hiring staff to deliver on his promises, not another chief spin doctor to put more gloss on the failings of his government."
Mr Carter, who got a CBE just over a year ago for services to the communications industry, will step down from his role at Brunswick with immediate effect.
He said: "It is an honour to be asked to work with the prime minister and the Downing Street team and I will do my utmost to make a difference to effective government."
Mr Brown knows the founder of Brunswick, Alan Parker, well, but the PM's official spokesman said he had not met Mr Carter, who will be based at No 10 and attend Cabinet, until recently
He left Ofcom in October 2006 before taking the top job at Brunswick Group, whose clients include: British Airways, BT, EMI, plus a number of small and unquoted businesses.
His new role comes with a salary of £137,000.
He will also step down as a non-executive director of Royal Mail Holdings plc, Travis Perkins plc and as a commissioner of the UK Commission for Employment & Skills.
He was part of the Capability Review Team that looked into the Department for International Development in 2006/7, and remains chairman of the Ashridge Business School and a governor of the Royal Shakespeare Company.
He is an honours graduate in law from Aberdeen University and a graduate of Harvard Business School's Advanced Management Program.