The adjudication, published today, relates to a prize draw campaign called The Alphabet Pack, which was sent out by WRVS in September 2011.
A supporter complained to the FRSB that the charity would not reveal how much it spent on the draw appeal.
The FRSB board ruled at the third and final stage of its appeals process that the appeal had breached the Fundraising Promise, under which members pledge to be open and honest with supporters.
It was breached because the WRVS failed to provide a clear and definitive answer to the complainant’s questions about the cost of the prize draw appeal to the charity, the board found.
The board also raised concerns that the WRVS did not refer the supporter who complained to the FRSB and alert them to its role as a regulator.
But it found that the charity’s actions had not broken the Institute of Fundraising’s Raffles and Lotteries or Direct Mail codes of fundraising practice.
The supporter had complained to the charity that the appeal was also pressurising and misleading, and raised concerns that the style of the appeal replicated bogus appeals . These complaints were resolved by the charity.
Colin Lloyd, chair of the FRSB, said: "When it comes to prize draws, particular care and consideration must be given to the exact wording used to ensure that recipients are absolutely clear how it works. Terms and conditions must be easy to read and the script must be of a reasonable size.
"With an increase in bogus activity and scams reported in recent years, it has never been more important that charities are open and honest, demonstrating best practice with clear fundraising materials, helping the public to make that distinction and to donate with confidence."
David McCullough, chief executive of WRVS, said: "This follows a long process during which the charity repeatedly attempted to resolve the matter with the complainant through written communication and with offers to meet in person.
"The charity prides itself on its high standards of fundraising and in being open and honest and has since undertaken a thorough review of its complaints procedure to ensure full compliancy with FRSB guidelines.
"WRVS is disappointed with the ruling, but we are always keen to improve our processes and grateful to the FRSB for the help it has provided in resolving this issue and ensuring we have the correct procedures in place."
The charity has put some frequently asked questions with information about its prize draw on its website, he said.
The FRSB said it asked WRVS to be clearer to its supporters about the cost of the appeal, which it has also done. The board said it was not its responsibility to review the cost of the campaign.
In February, the FRSB upheld a complaint against Unicef UK about a telephone fundraising campaign after the charity repeatedly telephoned someone who had asked it not to do so.
Over the past two years only two other complaints have been upheld. These were against Painted Children and Sports 4 All, which also became the first member in the FRSB’s history to be expelled for failing to hand over proceeds after an event.