Spencer sounded the alert after Icap reported revenues fell 14% in the six months to September compared with the same period last year.
This has been one of the toughest periods in my 36-year career in the wholesale financial markets, he says.
Trading volumes this year have fallen significantly across nearly all asset classes and geographies, whether equities, futures, FX, commodities, fixed income and OTC.
There was no shortage of culprits for the slowdown.
Spencer blamed a combination of global economic weakness, the eurozone debt crisis, bank recapitalization and deleveraging, uncertainty over regulatory reform, quantitative easing and near zero rates, to name the main ones.
He said, in the very short term, Icap was not anticipating any rapid improvement across its markets.
I do not believe this negative environment will continue indefinitely but equally I do not expect it to improve imminently, says Spencer.
It has been a time to weather a hard storm and prepare thoroughly for financial regulatory reform.
The call came as volumes on EBS, Icaps electronic FX platform, have dropped to the lowest level since the broker bought the business in 2006.
Average daily volumes (ADVs) in FX spot on EBS was $92.6 billion in October, down 17% from September and 46% down on October 2011. ADVs on EBS have fallen by more than 60% since peaking at $253 billion in March 2008.
Other leading FX trading venues also saw declining activity last month.
Once the market-leading electronic FX venue, EBS lost its crown to Thomson Reuters last November, while concerns about the activities of high-frequency traders on the platform prompted a review that resulted in a raft of changes to EBSs trading rules in September.
Those alterations have been unanimously welcomed by leading market makers, although Gil Mandelzis, EBS chief executive, told EuromoneyFXNews this week that the changes at the platform remain a work in progress.
Spencer said Icap had re-engineered itself towards electronic transaction and post-trade services, which he believed puts the firm in a very good position when markets do improve.
And although it is still early days, we are pleased that the changes we have recently made at EBS have been well received by our customers, he says.