UEFA have given FIFA a deadline of 2 November to release key documents and answer every question about its governance and decision-making.
At last Thursday's FIFA Council meeting, president Gianni Infantino apologised for his plan to sell the clubs taking part in the World Cup. He stopped short of explaining how the proposal came together.
In the letter seen by "BBC Sport", UEFA general secretary Theodore Theodoridis said world football's governing body had failed to address "a number of fundamental questions".
The statement read: "Other questions remain unanswered, and the information and documents requested have not been provided."
Running to eight pages, the letter covered everything from private investment in the FIFA Forward Enterprise (FFE) project to the FIFA Peace Prize and the controversial suspension handed to United States of America forward Folarin Balogun at the World Cup.
Theodoridis continued: "FIFA's withdrawal, the acknowledgement of mistakes, and the announcement of 'possible governance reforms' do not answer how the relevant decisions were taken or why the FIFA Council was not involved at the appropriate stage."
He went on: "Any refusal to disclose information or documents must be justified individually, with a clear explanation of the specific reasons relied upon."
A further passage read: "These questions require answers now. They cannot be postponed until the next meeting of the FIFA Council, nor can future governance reforms replace scrutiny of the decisions already taken and accountability for them."
What UEFA will do if FIFA miss the deadline remains unclear.
Infantino first faced the questions at the FIFA Council last week.
English Football Association president Debbie Hewitt, a FIFA vice-president, did not hold back afterwards: "If he does not answer these questions, he is not fit to be FIFA president."