Kristian Atkinson will support Sajiv Vaid as co-manager on the MoneyBuilder Income fund and the Short Dated Corporate Bond fund. Peter Khan will also support Sajiv as co-manager of the Fidelity Extra Income fund.
Tim Foster and Claudio Ferrarese will assume joint responsibility for the Fidelity Strategic Bond fund and Fidelity Funds (SICAV) Flexible Bond fund.
Spreadbury said: "I've managed real money for real people and it has been both a privilege and rewarding over my long career. I once said in an interview that the day you think you understand markets is the day you should retire. I'm not sure I can say I fully understand markets, but having managed money for more than 30 years, of which many have been here at Fidelity, I have decided that now is the right time to retire from fund management.
"I am particularly grateful to our investors who have remained committed to the funds over such a long period. We developed a consistent and disciplined investment approach, which will continue under the stewardship of Sajiv, Tim and Claudio. I have worked alongside them for the last few years and I am confident the funds will continue to deliver under the same entrenched philosophy."
John Clougherty, head of wholesale, Fidelity International, added: "Ian Spreadbury is one of the most experienced and highly respected bond managers in the industry. He has navigated his funds through the highs and lows of financial markets, delivering a consistent low risk approach to income investing.
"While we are, of course, extremely sad to see Ian leave, a long-term succession plan ensures that his funds will continue to deliver the same high quality outcome for our clients - remaining true to their objectives under the management of a highly experienced team including Sajiv Vaid, Tim Foster and Claudio Ferrarese.
"I would like to thank to Ian for his contribution to the success of Fidelity's fixed income business and wish him all the best in his retirement."
Ryan Hughes, head of active portfolios at investment platform AJ Bell, said the news was anticpiated and succession planning in place.
"News that Ian Spreadbury is retiring at the end of the year should come as no surprise given this has been well flagged by Fidelity for a number of years," he said.
"Succession planning clearly started three years ago with the recruitment of Sajiv Vaid from Royal London to co-manage the flagship MoneyBuilder Income fund and we have been hugely impressed at how Fidelity have handled this.
"Spreadbury has proven to be one of the most consistent fixed interest managers in the market and his calm and considered approach has seen the fund become a firm favourite for users of the AJ Bell platform.
"The Fidelity MoneyBuilder Income fund is used across the AJ Bell Active MPS range while the Strategic Bond fund sits on our Favourite fund list. With such strength and depth on the fixed interest team and a clear co-manager approach, I'm confident that Fidelity will manage the transition with aplomb and see little reason to change our view on both of these high quality fixed interest funds."