Sky News has learnt that Crystal Amber, which is AIM-listed Cenkos's fifth-biggest shareholder, wrote to Gerry Aherne earlier this month to urge him to launch a strategic review "to determine how value can best be realised".
The move by Crystal Amber, whose figurehead Richard Bernstein is a well-known activist investor, will ignite a battle over the future of Cenkos.
Mr Bernstein's letter is understood to have been sent to Mr Aherne a fortnight ago, before a warning from Cenkos was issued this week that full-year revenues would be "materially below" those of 2017.
The company's statement on Tuesday sent its shares plunging 12%, reinforcing the uncertainty surrounding many City brokers as new European markets rules inhibit the range of activities from which they can derive revenues.
Cenkos is no stranger to boardroom uncertainty, with previous rows having claimed the scalps of a number of directors.
In its letter, Crystal Amber, which is Cenkos's fifth-biggest shareholder, said Mr Aherne had failed to sufficiently active or engaged in order to build market confidence in the firm's prospects.
"We are concerned that the Cenkos franchise is struggling to build a broader, more recurring revenue base," Mr Bernstein said.
"Despite Brexit, London remains a pivotal financial centre.
"The existing platform can be an attactive springboard for a larger financial institution.
"Consequently, we propose that the Cenkos board should now commence a strategic review to determine how value can best be realised."
It was unclear on Thursday night whether Mr Aherne and his boardroom colleagues had discussed the Crystal Amber letter.
A major chunk of Cenkos's shares are held by employees, and it is far from certain that Crystal Amber would be able to force directors to conduct a strategic review without support from a large number of other independent shareholders.
The firm is one of the most prominent in the City, arranging company flotations, share sales and mergers including last year's listing of the logistics group Eddie Stobart and the 2014 initial public offering of the AA (Frankfurt: A116XA - news) .
However, it recently resigned as the broker to Stobart Group, the infrastructure conglomerate at the centre of a massive corporate governance dispute, and two years ago was fined for its work on Quindell (Berlin: 30382595.BE - news) , an insurance outsourcing business.
London's small- and mid-cap broking sector has been ravaged by new regulatory frameworks and declining revenues, with firms such as Oriel Securities, Panmure Gordon and WH Ireland (LSE: WHI.L - news) all either the subject of bids or speculation about their future as standalone firms.
A Cenkos spokesman declined to comment.