The Signal and Mentadent toothpaste brands, handled by the Lowe agency in Paris, are expected to be reassigned to Bartle Bogle Hegarty.
However, the agency would be forced to resign its Perfetti dental care account in Italy. BBH refused to comment.
The likely exits deal a further blow to Lowe, which last year lost Unilever's £8 million pan-European Surf business to BBH.
The agency is expected to have to make redundancies from its 270-strong workforce in London. Matthew Bull, the agency's chief executive, said: "There will be redundancies, but we don't know how many."
Lowe sources say that the redundancies are inevitable because of the scale of the HSBC business, which is said to account for 20 per cent of the London office's revenue.
The agency also lost the £40 million global Braun account to BBDO earlier this year. Unilever's £25 million global Flora business is also under review.
Privately, Lowe senior managers are calling on its Interpublic parent to reaffirm its faith in the network and end speculation that it could be merged with another of its operating companies.
Jerry Judge, the chief executive of Lowe & Partners Worldwide, claimed HSBC accounted for just 2.5 per cent of the network's revenue, "which we are already hard at work at replacing", and said he was unaware of Mentadent and Signal being reassigned.
Separately, Jeremy Bowles is quitting Lowe London, where he is the managing director. It is understood that he agreed his departure with Bull in March.