Mischief was brought in by Ben & Jerry's to handle consumer PR in late 2009.
Frederick's has claimed that the agency failed to disclose its work for Ben & Jerry's when the two parties met on 7 December and has stated that Mischief's behaviour is inconsistent with the PRCA professional charter.
But Mischief managing director Mitchell Kaye has hit back at the allegations, claiming the agency did not win the Ben & Jerry's brief until mid-December.
Frederick's creative director Matt O'Connor complained that he had 'disclosed confidential outline brand PR and marketing plans for 2010', which he followed with a detailed brief by email containing campaign ideas.
He claims the agency failed to reveal it worked for rival firm Ben & Jerry's before being party to confidential information about the firm.
'At no time did you disclose that there was a conflict of interest with a major competitor, either prior to our meeting, or before the brief was submitted. We were only informed subsequently in an email which stated that you were declining to pitch, citing an undisclosed conflict of interest,' wrote O'Connor in a letter to Mischief on 26 March.
But Kaye disputed this version of events when contacted by PRWeek.
He said: 'We pitched for Ben & Jerry's on 2 November and we won the business in mid-December. In that period, we were approached by Frederick's and met them for a chemistry session. By the time Frederick's asked us to pitch we had been hired by Ben & Jerry's and therefore declined their invitation.'
Kaye also questioned O'Connor's claim that Mischief refused to sign a non-disclosure agreement (NDA), arguing that Frederick's did not provide an NDA to sign. He added that the agency has signed one retrospectively.