Trustees of the YMCA's English, Scottish and Welsh councils, who run the final-salary pension scheme through an organisation known as YMCA Pension Plan Trustee, are asking local members to increase their contributions.
Hastings and Rother YMCA, which would face an annual bill of about £14,000 under the plans, is leading a revolt of about 10 local YMCAs.
The YMCAs are unhappy about YMCA Pension Plan Trustee's "heavy-handed" demands for money and the way the fund has been run.
They plan to hold a meeting before the end of the month to decide how they will tackle the situation.
"We were told that if we did not pay we would be reported to the Pensions Regulator and trustees would be personally liable for the full amount of £400,000," said Wes Jefferies, vice chair of Hastings and Rother YMCA. "Our turnover is only about £150,000 and we do not make much of a surplus."
A letter to lawyers acting for the fund from John Quarrell, Hastings and Rother YMCA's solicitor, alleged that the plan had been "incompetently managed".
"In almost 40 years of practising in pensions law I have never come across adherence to a pension scheme in such an ad hoc manner," Quarrell wrote.
The letter also criticised the "odious and objectionable" manner in which demands for money from the individual YMCAs had been made. "If there is a real problem, which it is clear there is, then threatening elderly widows and decent citizens with bankruptcy is not a solution," it said.
Paul Smillie, company secretary of the YMCA Pension Plan Trustee, said the fund had not been badly managed and the majority of employer members were happy to make their liability payments.
"I personally believe we have not made threatening demands," said Smillie. "The fund has to comply with pensions law. If organisations do not pay, that becomes a reporting requirement."
The pension body said it had made numerous offers to meet Hastings and Rother YMCA to discuss the situation but that these had not been taken up, added Smillie.
The pension plan is run by YMCA Pension Plan Trustee and was started in 1945
There are 148 participating employers across England, Scotland and Wales
Forty-eight YMCAs have been calculated as having liabilities by the pension company
The plan closed to new entrants in May 2007.