Precise details of the deal aren't yet available, but the bank in question is likely to be named in the next few weeks.
Getting a major financial institution to ditch Microsoft Windows in favour of JDS is something of a coup for Sun, but it may not be a major money-spinner.
"I hope they won't be a template for future customers," said Robin Wilton, Sun's JDS programme manager for Europe, the Middle East and Africa. "Boy, have they put pressure on us for a good price."
The bank in question is using the Java desktop software as the foundation for a customised desktop platform for its staff, which Wilton describes as a "rich banking client".
Ironically, this will involve removing some of the functionality that Sun has included in JDS to compete with the like of Windows, a process Wilton described as "deintegration".
"They want us to rip out the instant messenger client, for example, so they're left with a robust core that needs minimal maintainance."
The JDS rollout will supersede a Windows 3.1 suite which the bank was writing off over a ten to 15 year timeframe -- which is an indication of its thrifty approach to IT.
"If all companies are like this, it really will be earning a fiver the hard way," joked Wilton, in a speech at the Linux User and Developer Expo 2004 in London on Tuesday.
This isn't the first time that Sun has secured a JDS deal that won't yield massive profits. It recently secured a deal with the Chinese government that could see hundreds of millions of PCs running desktop Java, but chief executive Scott McNealy has admitted that this won't be a big money spinner. McNealy told a Sun European user event in Berlin last December that the agreement was a strategic play that stopped Microsoft dominating the lucrative Chinese market.