Rathbones Group is to buy the UK wealth management arm of Anglo-South African bank Investec for £839m.
Rathbones aims to achieve £60m of annual synergies, partly through combining technology and shrinking the property portfolio.
The all-share merger will create a London-listed wealth management group with about £100bn of funds under management.
Investec will own 41.25% of the combined group, although its voting rights will be capped at 29.9%.