Dame Lynne Owens has been appointed the interim deputy commissioner of the Metropolitan Police.
She was previously the head of the National Crime Agency before stepping down last year to focus on her treatment for breast cancer.
Dame Lynne was considered a leading candidate for the role of Met Police commissioner, before pulling out of the race ahead of the application deadline, and the job instead went to Sir Mark Rowley.
At the time she tweeted that she would not explain her decision except to say it was not health related.
She will hold the interim position of deputy commissioner for six months while a recruitment process is run.
In a statement, Dame Lynne said: “I am enormously pleased to have this opportunity to return to the Met to support Sir Mark in delivering his plan for London as well as its communities and its officers, staff and volunteers.
“His intent is undoubtedly the right one, requiring a team effort to renew policing by consent with less crime, higher standards and more trust. I can’t wait to get started.”
Incoming Commissioner Sir Mark hailed Dame Lynne as a “outstanding leader”.
“Lynne is exceptional,” he said.
“She is an outstanding leader, who brings immense operational expertise and experience driven by the value of public service.
“She is a dynamic chief officer who is prepared to confront challenges and is passionate about the relationship between police and communities.
“We are fortunate to have her join the team.”
Home Secretary Priti Patel said that Dame Lynne was the “right person” to support Sir Mark as he starts his new role.
“Dame Lynne Owens is a strong and formidable leader with decades of experience in law enforcement and I am confident she is the right person to support Sir Mark as he starts his new role as commissioner,” she said.
“Rebuilding public trust in policing, tackling neighbourhood crime and getting the basics right must be the top priorities for the Met’s leadership team.”
Mayor of London Sadiq Khan also welcomed her appointment, adding that her knowledge of the Met will be “invaluable”.
“Dame Lynne brings decades of experience, from her senior roles in the Met, Surrey Police and most recently as lead for the National Crime Agency,” he said.
“Her knowledge and insight outside the Met will be invaluable in the coming months as the Met focuses on change and improvement.”