Prince Harry said the best birthday present the Queen could get would be a "day off so she could lie about and do nothing".
The Queen's grandson chatted animatedly to guests at London's Guildhall at a reception marking her milestone anniversary.
Grenada High Commissioner Karl Hood spoke to Harry about the Queen's official 90th birthday celebrations in the crypt of the City of London's historic building.
"He said the best birthday present would be to have a day off, so she could lie about and do nothing," Hood disclosed.
Acting High Commissioner for Dominica, Janet Charles, revealed that Harry said the Queen should be putting her feet up, but had wanted to attend the service of thanksgiving in St Paul's Cathedral for the public.
"He was talking about his grandmother. He said she should really be putting her feet up but she wanted to be here for the people," Charles said.
"He said he hopes there's many more years of that to come."
She added: "He's such a charming person. I asked him if I could have a photograph with him he said he couldn't as he had to obey the rules.
"But he said 'I'm sure we'll meet again'."