Houghton Mifflin, the US educational publisher, has disputed a ratings agency downgrade for the second time in two months, heightening debate about the indebted group’s outlook.

Standard & Poor’s last week cut its ratings for HMH and Education Media & Publishing, the private parent company created from $7.4bn of acquisitions made in 2006 and 2007 by Barry O’Callaghan, the founder of Riverdeep, an Irish software group.