E-Clear, the firm that had processed the budget flier's online payments, said it was appalled by the behaviour of accountants PricewaterhouseCooper (PwC).
The chairman of its advisory board, former Tory grandee Sir Edward du Cann, accused the accountants of encouraging other firms to criticise E-Clear.
The robust defence comes just days before E-Clear faces a legal bid by PwC to force it in to administration.
PwC yesterday said its action at the High Court was a "last resort" in a bid to secure tens of millions of pounds it believes belongs to Flyglobespan and its Edinburgh parent company Globespan.
Du Cann yesterday told Scotland on Sunday: "I am appalled at how E-Clear has been attacked by companies who are not telling the full story.
"I am also unhappy with the administrators PwC. They have encouraged companies who have faced problems through their own failings to jump on the bandwagon and criticise E-Clear.
"This we will not do. Globespan would have closed in 2007 without E-Clear."
Bruce Cartwright, Globespan's PwC administrator, hit back, saying: "Clearly, it is regrettable that Sir Edward blames E-Clear's problems on Globespan's failure.
"The fact of the matter is that E-Clear were provided with every opportunity to cooperate with the Globespan administrators, PwC, and the instigation of legal proceedings was a course of last resort and a result of E-Clear's failure to cooperate."
E-Clear has been widely accused of forcing Flyglobespan in to difficulties by holding on to millions of pounds of money from customers.
A High Court judge ordered E-Clear to provide evidence by lunchtime on Friday that it still holds that 35m.
A court hearing will resume on Tuesday.