There is growing concern among policymakers and analysts that the true extent of European banks' debt problems is being masked.
His stern warning to U.K. banks that they need to drop the "pretense" that some of their bad debts will be repaid was coupled with the statement that they have "insufficient capital" to deal with losses which have remained undeclared.
Essentially, what seems to have happened is that banks across the euro zone have put off foreclosures on weak businesses – a process known as forbearance. This has been enabled by low interest rates across the region and rescue packages which have injected unprecedented amounts of liquidity into the banking system and helped keep struggling economies afloat.
The scale of forbearance is hinted at in relatively low rates of company insolvencies.
In the U.K., despite the recession, insolvency rates are similar to 2002, when the economy grew by 1.6 percent, according to government figures.
(Read More: Is It Time to Kill Off U.K.'s Zombie Companies?)
Greece's problems have been well flagged – yet just five Greek companies were declared insolvent in 2011, the year it was forced to seek a bailout from international lenders, fewer than in 2007, when its economy was still growing.
This persists across the euro zone, with the weakest economies sometimes experiencing its lowest insolvency rates.
In 2011, the number of insolvencies per 10,000 companies was lowest in Greece, Spain, Italy and Portugal, according to calculations from Creditreform.
However, as Nigel Myer, director of credit strategy at Lloyds, pointed out, the extent of this is "effectively invisible" and "almost impossible to quantify." Decisions are made by individual banks and they do not have to declare them under accountancy rules.
Putting off foreclosure could be dangerous not only because it masks the true state of businesses, but because it could mean a faster rate of insolvencies if banks decide to change their policies in response to a worsening economy, with potential damage to employment figures and the broader economy – and to the banks themselves.
"To the extent that forbearance has taken place, a worsening economic environment in these countries could lead to a faster rate of deterioration in asset quality than might be inferred from reported numbers," Myer warned.
(Read More: 2011's Biggest Bankruptcies)
Of course, delaying the repayment of non-performing loans can be positive for the economy, particularly in the short-term.
"It has allowed companies to survive and people to be employed," as Myer pointed out. "It also very likely supports tax receipts and reduces the need for social security support."
Sir Mervyn's warning does not chime with other influential figures in the UK.
The European countries least likely to be affected by forbearance following worse-than-expected economic data are Switzerland, Austria and Denmark, according to Myer, who suggested spreads in Swiss banks and the recent rally in Danish spreads should be supported by worries about forbearance.
Written by Catherine Boyle, CNBC. Twitter: @cboylecnbc.