Scott Gallacher, director at Leicester-based Rowley Turton, said he has experienced a problem with Sun Life Financial of Canada’s approach to letters of authority.
Mr Gallacher said the provider refuses to talk to anyone except the adviser named on the letter of authority signed by his client.
A spokesperson from one well-known provider, who did not wish to be named, said this was not normal practice and when a letter of authority is signed they "release information to anyone in that firm on the case, not just one adviser".
Neville Kent, chief operating officer at Sun Life of Financial of Canada, defended the company's position.
Mr Kent said: "We work on a letter of authority basis if not dealing with the client themselves. Typically those letters of authority are issued for a specific adviser.
"We provide information in accordance with the letter of authority we have. If it specifies that information we provide that, if it is broader then we would provide that.
“We only provide information in accordance with the authority we have from the customer."