The officers checked the true weights against records kept by the club - and many members found they were heavier than they had been told.
Divisional officer Pete Tomkins, of Dudley Council Trading Standards confirmed the investigation was taking place following a complaint. He said: "We have not yet interviewed the person who runs the club. A considerable number of members were weighed by our officers and comparisons are being made of the weights of individuals as recorded by the club and by our staff."
Mrs Elaine Smith, who runs the club at Stourbridge Town Hall, West Midlands, said: "I am very upset by the whole thing. This affair is most baffling.
"The scales I use are of a medical type. I have been working with similar scales now for 20 years and I have never had any problems with them before." She added that the scales were calibrated every year by weights and measures officials.
"My whole reputation is being strict and methodical. I have nothing to gain by putting down inaccurate figures."
One member said: "The trading standards people said I was about five kilograms heavier than the figure the club had put in my booklet."
Another, praising Mrs Smith's "tremendous leadership abilities" said: "many of the members worship her".
Trading standards officials have told Mrs Smith it would be about a week before they knew whether any action would be taken.