A disorderly end to the turmoil would "inevitably" lead to a global recession, crushing companies' profitability, they believe.
"Global equities could lose up to 35pc of their value if the situation deteriorates into a full-blown financial crisis on the scale of the fallout from the collapse of Lehman Brothers in 2008," the analysts said in a note.
Even in the "milder" scenario where the crisis does not spill over into the wider economy, would see markets fall by 12pc.
The warning came as EU president Herman Van Rompuy said eurozone nations will hold a special summit in Brussels on Thursday over a second bail-out for Greece.
The announcement came after the International Monetary Fund urged officials to act with a "greater sense of urgency".
The flight from Italian and Spanish bonds in recent days has stoked fears that countries "too big to bail" could become embroiled.
Division in the eurozone over to how to tackle its debt woes were thrown into sharp relief by statements from Germany's finance ministers on whether the shared currency is under threat.
There is "no crisis of the euro", according to Germany's deputy finance minister, Joerg Asmussen, calling it "a crisis of public debt and competitiveness".
Meanwhile Wolfgang Schaeuble, his finance minister, told a newspaper: "The crisis in confidence ... prompted by Greece is now endangering the euro as a whole."
303 Comments