The merger, which is expected to be announced imminently, would end years of speculation over the future of the two agencies.
TBWA\GGT only operates in the UK, while Tequila has a global presence as TBWA's below-the-line network. A merger has been considered previously but issues such as client conflict have stood in the way.
The merged agency would keep the Tequila\London name and sources suggested that it would be led by Tequila's management. However, TBWA is understood to be considering the details of senior management roles and client issues.
It is unclear which positions will be offered to TBWA\GGT's chief executive, Mike Cornwell, and the managing director, Penny Reid, and Tequila\London's chief executive, Tim Bonnet.
Paul Bainsfair, the president of Europe for TBWA Worldwide, said: "This is an issue that has been under consideration before because both agencies occupy similar parts of the market. At the moment the two companies stay separate."
Financial services clients could present conflict issues with TBWA\GGT handling Abbey and Prudential and Tequila holding Barclays and The One Account business.
However, TBWA\GGT's loss of its Vauxhall account earlier this year, a potential clash with Tequila's Nissan business, has eased that conflict issue.
Cornwell said: "This is news to me and has been news to me every year for the last six years when somebody has asked me the question. Why would the group do that when it has two interesting companies?"
Bonnet said that there were "no fixed plans" to merge the agencies.