The decision comes only four months after the high-street banking giant appointed Joshua to its £5 million business banking and savings account.
TBWA/GGT originally won the £20 million account in the summer of 1998, following the bank's decision to streamline its advertising roster. At the time, it was the biggest direct marketing win in UK advertising history.
It is currently unclear as to what caused the split. However, the move is not thought to affect either M&C Saatchi, NatWest's creative agency of record, or its media agency, MediaCom.
Earlier this year, NatWest's parent company, the Royal Bank of Scotland, announced its intention to spend £150 million re-branding its 1,650 NatWest branches.
This programme spelled the end for the company's famous black-and-white logo, which is being replaced by a new corporate identity, developed by the brand consultancy The Partners, which comprises pink and blue shades. The identity will be replaced in all NatWest branches by the end of the year.
The rebranding was part of RBS's plans to give NatWest a more consumer-friendly image and arrest its decline in customer numbers.
Since it acquired NatWest in 2000, RBS has abolished plans to close numerous branches and has brought in an extra 1,000 customer advisors. In addition, it has promised small businesses that no account manager will be moved in less than four years.
No-one at TBWA/GGT was available to comment as Campaign went to press.
NatWest refused to comment.