Royal British Legion uses gagging clauses in contracts
The Royal British Legion is using so-called gagging clauses in contracts for some staff leaving the charity.
The organisation says 26 former employees have signed such agreements over the last 10 years.
Public Concern at Work, a group which backs whistle-blowers, called the practice "unacceptable".
Although there have been no allegations of misconduct by the Legion, such clauses have proved controversial in recent years.
The Legion, which provides practical, emotional and financial support to servicemen and women, their families and veterans, said it used them "very rarely".
It pointed out that only 1% of departing staff have signed the agreements in the last 10 years.
They have been widely used in business and until recently in the NHS.
They impose a duty of confidentiality on both the staff member and employer.
The Public Interest Disclosure Act does protect those who breach the clause to report issues of public concern.
But Nick Hine, an employment lawyer, said despite that, there is a powerful incentive not to speak out.
"If they don't adhere to the agreement, the risk is that they have to pay back the money, or some of the money.
"The reality is that people don't talk about it."
But, following the scandal at Stafford Hospital, when staff said they were too frightened to question patient safety, the clauses were in effect banned across the NHS.
The support group Public Concern at Work believes the clauses are also not appropriate in the voluntary sector.
Chief executive of PCAW Cathy James said: "They're dangerous, because there's a real risk that public interest information is buried in settlements.
"And, where we're talking about a large public institution with a huge amount of public trust, that's simply not acceptable."
Conservative MP Bernard Jenkin, chairman of the Public Administration Select Committee, believes that charities do have every right to demand confidentiality.
But they have a special relationship with the public, he said.
"Because of their privileged tax status, because being a charity is meant to be for the public benefit, they have a special obligation to be open and transparent."
The Legion insists that nothing in the clauses prevents someone from talking about issues of public importance.
That position is supported by the Charity Commission itself which said it would only be concerned if there was evidence such clauses were being abused.
But the revelation about the gagging clauses comes at a time when some members of the legion are questioning the way it is being run.
At the charity's annual conference this weekend, one branch is planning to put forward a motion to demand members have a greater say in its running.
Steven Bonde, membership secretary of the Stamford Branch in Lincolnshire, said: "In any other public organisation the shareholders have the right to hold directors to task, we have no right."
John Copeland, the chairman of the same branch, said it was impossible to find out basic information.
"We need change for the better, with the membership, which is the whole ethos of the Royal British Legion. We love the legion."
The Legion insists ordinary members are "fairly represented in a democratic manner".
The organisation, which has 340,000 members, spent £84m last year on the health and welfare of the armed forces and their families, according to its website.