Cooper Barry says that it has used "Now there's a thought" extensively for around five years since it launched a new identity and corporate strategy in 2001.
The firm is seeking a High Court Injunction tomorrow, Friday, to attempt to bring the high street bank's ad campaign to a halt.
Barclays is due to launch its most extensive ad campaign in years on October 21 during 'Coronation Street' on ITV1. The campaign has been created by Bartle Bogle Hegarty and is supported by national press and outdoor ads.
Cooper Parry was first alerted to the group's ad campaign two weeks ago. It has since attempted to get the bank to rethink its strategy, arguing that the new campaign is a threat to its independence and could fundamentally damage its goodwill.
Jeremy Bowler, chief executive of Cooper Parry, said: "Barclays has made no attempt to recognise the magnitude of the impact on our business.
"I am simply devastated that a bank, which purports to have such a high regard for its corporate responsibilities, should act in this way. We are left with no choice but to stand and fight to protect our business."
Barclays said it listened to the concerns of Cooper Parry with the aim of coming to an agreement whereby both companies could continue to use the strapline.
In a statement, the bank said: "We are disappointed that we have been unable to resolve the situation without Cooper Parry feeling the need to resort to legal action. We will now have to wait the decision of the courts on Friday."
The new Barclays campaign moves away from its previous "Fluent in finance" strategy, starring Samuel L Jackson among others, and has set its sights on showing customers all the different ways Barclays can help its customers.
The first ad to air is called "invisicard", and focuses on how Barclays staff develops a new invisible card. While celebrating, the card is knocked on the floor and lost. The ad highlights the point that while an invisible card might not be here yet many anti-fraud measures are in place.
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