The union has refused to confirm its membership numbers

Union Ucatt has refused to confirm its membership numbers amid accusations it has more than doubled its figures to increase its clout.

The controversy arose after a watchdog declared the union’s 2009 leadership election void in March, after it found ballot papers were sent to less than half its claimed membership.

The union later claimed the 70,000 members who were not sent ballot papers were not in fact members of the union - a submission the watchdog threw out.

This week the union came under additional pressure to justify its payments to the Trade Union Congress (TUC), made on the basis of a claimed membership of 128,964.

The affiliation fee cost Ucatt about £300,000 last year, but critics claim the union is overpaying the TUC out of “vanity”, on the basis of inflated membership figures.

Mick Dooley, who successfully overturned the 2009 leadership contest in which he was defeated, said: “This money should be being used to look after union members.”

Ucatt declined to comment but has previously said that its rules allow it to exclude members who are over 26 weeks behind in membership.