The battle between the UK's trade unions for a share of a declining membership has intensified, with the GMB fighting a bitter equal pay dispute against Amicus.
Several claims by GMB members employed by Amicus are being investigated.
In a document leaked to Personnel Today, the GMB accuses Amicus - the UK's biggest private sector union - of "hypocrisy" by levelling down salaries of its own staff in one equal pay claim, rather than lifting pay to the higher comparator.
The allegations are based on the case of a female Amicus staff member who put forward five other staff members as comparators to show she was being paid less for work of the same value. The union's response was to send a "notice of termination" to the comparators - signed by general secretary Derek Simpson - offering new terms based on a lower salary, and blaming the GMB for forcing it to "strictly apply the law".
The GMB officer in charge of the claims, Dave Kent, would not give any further details, but said the union would support its members "wherever they are being mistreated".
A spokesman for Amicus dismissed the GMB's claims as "spurious", and denied there was any policy of levelling down pay. He claimed the letters of termination were "the start of a consultation period" on pay levels. But he conceded the consultations could lead to drops in salaries, and Amicus had "a duty to run a solvent organisation".
The leaked document also raises questions about what will happen to the pay of other staff when Amicus merges with the Transport and General Workers' Union (T&G) next year.
"We are sure all employees are mindful that this has severe implications, particularly in view of the merger with the T&G," the document states. "Where pay differentials exist, will they harmonise at the higher or lower rate?"