11/29/2006 AT 03:00 PM EST
The top sport in England these days isn't football or polo – it's guessing when Prince William will finally get engaged to his longtime girlfriend Kate Middleton and start the countdown to what would be the wedding of the century.
Now into their fifth year as an absurdly attractive couple, Will and Kate, both 24, have lately seemed so snuggly – and Kate so seamlessly ingrained into Will's inner circle of friends and family – that the engagement gossip is getting as hot as a pot of Earl Grey.
Britain's Mail on Sunday even reported last week that Kate has been invited to the Queen's country retreat Sandringham to join the entire Windsor clan for Christmas Day lunch – an unprecedented honor for an unmarried partner of a royal and, if true, said one royal observer, tantamount to an engagement announcement. (Buckingham Palace won't confirm the invitation.)
There is also the 10,000-sq.-ft. neo-Georgian mansion being built by the royal family on part of a 900-acre estate in a village in Herefordshire, which many believe is being readied for Will and Kate.
"It's decision time," says Robert Jobson, author of a new book about the couple, William's Princess. "Will has said he doesn't want to get married until he is 28, but that is just a smoke screen. The pressure starts now."
So just how close is Will to popping the question? Those in the know at Buckingham Palace say no one is ordering "Save the Date" cards just yet and insist Kate isn't being formally groomed for the future.
"There are no people advising her or preparing her for anything," says one palace insider. "She is going about her private life as a private individual."
Friends of the couple say that Will, known for his stubborn streak, won't let anyone dictate a time frame – but also acknowledge an engagement is no longer a matter of if but rather when.
"This is certainly not something they are going to rush into," family friend tells PEOPLE. "But their relationship is very strong."
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