LONDON — Fallen banking titan Bob Diamond on Wednesday described regulators on both sides of the Atlantic as partly complicit in a scandal involving the manipulation of a key interbank lending rate, telling a British parliamentary committee that government watchdogs had failed to act after his bank, Barclays, informed them of industry-wide irregularities during the U.S. financial crisis.

The allegations highlight the relationship between financial institutions and regulators at a time when risk-taking and misdoings at big banks again are the focus of debate in Washington, on Wall Street and in London. Diamond stepped down Tuesday as chief executive of Barclays on the heels of a U.S.-British investigation that resulted in a $450 million fine for the bank for manipulating key lending rates between 2005 and 2009. The resignation ended the tenure of a legendary figure who brought a high-flying American-style culture to the bowler-hatted bankers of London’s financial district.

Barclays is one of a number of global banks being investigated for alleged improprieties that tainted the credibility of the London interbank offered rate, or Libor, the benchmark figure that largely determines the adjustable lending rates for U.S. credit cards, student loans and some mortgages. The emerging scandal has touched off a firestorm engulfing the London financial world, with Prime Minister David Cameron this week announcing a broader inquiry into banking standards that is set to haul some of the globe’s most powerful financiers before a parliamentary committee.

At the hearing Wednesday, furious British politicians seemed to put Diamond on trial as if he were the Gordon Gekko character from the “Wall Street” films, blaming him for importing a culture of risk and big bonuses to London. The 60-year-old American sought to defend his response to the Libor scandal, insisting he learned only last month about the extent of wrongdoing among an “abhorrent” but “small” group of 14 rogue traders at Barclays who had been manipulating rates for personal gain.

Diamond also painted a picture of the bank’s accusers — government regulators — as at least partially to blame. Documents released by Barclays on Tuesday night said the bank had “raised concerns” with British regulators, the Bank of England and the U.S. Federal Reserve that other financial institutions were not being honest about interbank lending rates during the financial crisis that peaked in September 2008.

Other banks, Diamond said, were routinely underreporting the rates at which they were borrowing, afraid that revealing how high their costs had soared would spark an investor panic or government nationalizations. He seemed to suggest that regulators were content to see misreporting of interbank lending during times of crisis, when strict accounting of high rates could tighten lending even more, and that they acted to curb such activity only during less sensitive periods.

Asked how regulators responded to Barclays’s reports of widespread misreporting, Diamond said, “Various levels of acknowledgment but no action.”