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It's every bride and groom's worst nightmare: A rainy wedding day!

And, right now, the forecast in London on Friday, April 29, predicts a 60% chance of rain (according to

Indeed, a palace insider says that tensions are running high at Buckingham Palace that Kate Middleton and Prince William's big-time wedding could be spoiled by a downpour.

"We are worried about the rain on Friday," palace worker George Martin tells Us Weekly exclusively. To cope with that wet possibility, "[We] have made sure to have plenty of umbrellas on hand," Martin added. "It's the last thing we would want spoiling the day."

Adds the palace insider: "Hopefully the rain will stay away!"

PHOTOS: How Kate and William stepped out after their engagement

Rain or not, a palace aide tells Us that Middleton, 29, is "feeling very positive" as her wedding approaches. "Two months ago, Kate was full of nerves, but she's over that," the insider says. "She's so excited -- like a child waiting for Christmas, counting down the nights until she can open her presents!"

For more on the Royal Wedding, including rare photos, interviews with family and friends, details on Kate's new life as a princess and a tour of their lavish homes, order a copy of Us Weekly's Will & Kate: The Royal Wedding now.