Bank of Scotland is aligning all its £30 million advertising and marketing activity within the WPP group, which will co-ordinate it through Conquest.
The assignment effectively doubles the size of the London shop, which triumphed in a 'winner-takes-all" pitch between the between the bank's roster agencies.
The pitch process, which began nine months ago, is part of the bank's attempt to raise awareness through a more coherent approach to the management of its brand.
Conquest won through against McCann-Erickson, which handles Capital Bank, the Bank of Scotland's finance house, Bartle Bogle Hegarty, which runs its Investors' Club account, and the Leith agency in Edinburgh, which promotes the bank's branches.
Media arrangements remain under discussion. Some is currently bought and planned through Conquest with Feather Brooksbank handling the rest.
Martin Sorrell, WPP's chief executive is understood to have been closely involved in the pitch, whose result means a huge jump in Conquest's Bank of Scotland billings which currently stand at about £7 million.
The win climaxes a four year relationship between Conquest and the bank which first awarded the agency its direct account, later following up with its credit card business.
It also marks the first major success for Conquest's new positioning as a premium-priced stratregic advice specialist and a "gateway" to WPP specialists ranging from direct marketers to brand identity consultants and PR advisors.
The account is expected to be handled by a dedicated core group within Conquest which will draw on other WPP group resources as necessary. The agency is also expected to hire extrea staff.
The review of the bank's agency relationship is believed to have been precipitated by the joining of the marketing-orientated Capital Bank with the more traditional main banking business.
Now it wants to replicate its high awareness within Scotland south of the border.
Some experts believe that, having been thwarted in its attempted takeover of NatWest, Bank of Scotland could itself be vulnerable should its arch rival, the Royal Bank of Scotland succeed in its £26 billion hostile bid for NatWest.