The bank is holding talks to three agencies. Lida and Joshua, which already work with NatWest, will compete against the non-roster agency Kitcatt Nohr Alexander Shaw for the business.
Mortgage activity is handled by a number of roster agencies and NatWest, owned by the Royal Bank of Scotland, wants to consolidate the business into one agency.
The pitch is the latest stage in a gradual strengthening of the bank's roster. Joshua and Craik Jones Watson Mitchell Voelkel joined the roster in 2003, when the business moved out of TBWA\GGT.
Tullo Marshall Warren was added to the roster last year to handle relationship marketing for its FastPay online payment system.
If you have an opinion on this or any other issue raised on Brand Republic, join the debate in the Forum here.