Deregulated post carrier TNT Mail UK has secured a contract with Lloyds TSB to handle more than 100 million items of mail.
The bank will use TNT Mail UK to carry out 48-hour upstream mail services and the Royal Mail will carry out "final mile" delivery to individual households.
No value was given for the contract but TNT said it was one of the largest to be awarded so far in the newlyderegulated postal market.
Lloyds TSB joins other TNT Mail UK clients such as Sky, Caudwell Communications, Express Gifts, Booker and others operating under TNT Mail's access agreement with Royal Mail.
Mail UK said the deal should take volumes for 2005 to more than 500 million.
Managing director Nick Wells said: "This is a major win for us and we hope to grow our presence in this sector."
Mr Wells said businesses could not afford to ignore the deregulated postal market.
"Finance directors of all companies should be looking at how we can make their mailing budgets work harder for them," he added.