Bank of America was directly involved in markdowns that contributed to Merrill Lynch’s $15.3bn loss in the last quarter of 2008, its final reporting period before the Wall Street bank was acquired by BofA, sources familiar with the matter say.

Mounting losses at Merrill during December almost derailed the acquisition. Ken Lewis, BofA’s chief executive, threatened to walk away from the deal unless the US government provided $20bn in extra capital. The deal closed on January 1 after federal officials pledged their support.