Elsen, who left in October, is understood to have started employment tribunal proceedings against the firm, regarding alleged constructive dismissal.
He is also believed to be looking into further legal action over a claim of breach of his contract of employment.
His departure follows what one source describes as a 'difficult board meeting' in October.
He is the third out of the four original founders to leave the firm.
CEO Stephen Lock stepped down in April this year, following the departure of finance director Sunil Sharma.
The eight-strong agency includes MD Jeremy Swan and the only remaining founder, Iain Anderson.
Anderson confirmed the matter was with his company's lawyers and refused to comment further for legal reasons except to say the firm had a duty to shareholders to protect its rights, 'through a tribunal or through the courts'.
It is understood there are no immediate plans to replace Elsen, with the firm concentrating on PA and policy work in Westminster and Brussels as well as healthcare.
Elsen is in the early stages of setting up a new litigation PR firm.