James North, chief of staff to Birmingham City Council leader Mike Whitby and an influential figure at the heart of the Tories in the ruling coalition, has quit his £45,000-a-year job.
He is to take up a post in the new year as senior programme manager to the Lord Mayor of the City of London.
Mr North, who has been at the council leader's side for three years, said he had learnt a lot and would have fond memories of Birmingham, but the time was right to move on.
The 27-year-old high-flying graduate of St Andrew's University attracted a reputation for sharp dressing, with his trademark braces, pink shirts and colourful socks. But behind the cut-glass vowels and flamboyance, there was a sharp political brain and an unnerving ability to protect Coun Whitby from most pitfalls and intrusive media interviews.
As the council leader's "gatekeeper", North became a first port of call for anyone wishing to meet Coun Whitby, much to the surprise and even resentment of senior figures from the world of business and politics who found themselves having to negotiate with one of relatively tender years.
He played a major role in researching and organising the council leader's trade missions to China.
Mr North stressed there was no question of him having "fallen out" with anyone in Birmingham. He was leaving to enhance his career opportunities.
He added: "Mike and I had an open and honest discussion. There certainly wasn't any problem here. The time was right for me to move on."
In his resignation letter, Mr North spoke of his great sadness at quitting and said he would be leaving with a "heavy heart".
The letter added: "It has been an immense privilege and pleasure to work for you over the last two years. I have greatly enjoyed the experiences and opportunities this role has given me, working alongside you as you lead the city council forward on its journey to excellence, delivering improved services and strong fiscal management.
"It has been an honour to be part of the team delivering this change. This is an excellent city with an exciting future and I will be very sad to leave it."
Mr North arrived in Birmingham in December 2005, having been recommended by the then Conservative Party Chairman Francis Maude.
A former Tory candidate for the Scottish Parliament, Mr North decided against a career as a politician in favour of working for the Conservatives behind the scenes.
His impact on the council in such a short space of time led to him entering The Birmingham Post Power50 list, as the 40th most powerful and influential person in the city.
In his new role, Mr North will be responsible for organising more than 25 foreign business tours by the Lord Mayor of London, who acts as the UK's ambassador for the financial services sector.
"I am being completely selfish in leaving Birmingham. It is a great next step forward, Mr North added.