Sue Gray will not take up her post as the prime minister’s envoy to the nations and regions.
She was initially appointed to the role after departing as Sir Keir Starmer’s chief of staff last month, saying she “risked becoming a distraction”.
The government had previously said she was taking a short break before taking up her new position.
A friend of Gray said: “Sue has taken a decision not to take the role. She’s going to focus on other things.”
A Downing Street source told the BBC: “We think she has made the right decision.”
The news she would not take up the role was first reported by the Financial Times and the Guardian.
Gray left as the PM’s chief of staff after just three months in the role, following weeks of negative headlines and briefings against her, including a row over her salary.
She was replaced by Morgan McSweeney, with whom she had reportedly clashed in his previous role as Sir Keir’s chief political adviser.
Gray, who became a household name after leading the government’s internal inquiry into the Partygate scandal, left the civil service to become a senior adviser to Sir Keir last year.
