Citywire AA-rated St John will take the lead on management of the portfolio from early next year.
Thomas has led the fund for 16 years. During his tenure it has returned 380%, beating a UK All Companies peer average of 237%.
Global head of AXA IM Framlington equities Mark Beveridge said: 'Nigel Thomas has an impressive 39 years working in asset management and 16 years at the helm of the hugely successful Axa Framlington UK Select Opportunities fund.
'We are looking forward to continuing to work with Nigel over this coming year, but would like to take this opportunity to sincerely thank Nigel for his time at the company, his dedication to his unit holders and to the UK equities desk. We wish him all the very best for the future as he enjoys his retirement.'
Thomas originally trained as an accountant with Robson Thodes and then moved to Carrington Pembroke, now known as Artemis.
He joined the company in 2005 from F&C Asset Management.
Thomas said: 'I have worked with Chris St John for 13 years and we have very similar investment styles; he and the wider UK equities team have contributed a huge amount to the idea generation that I have implemented as I managed the UK Select Opportunities fund.
'Chris’s experience investing in UK small and mid-cap companies is particularly relevant as he takes the lead on this fund later this year.
'I have 70% of my SIPP invested in the fund and plan to keep it there - testament to my faith in Chris to continue to deliver superior returns for clients.'