Bookies slashed odds on Prince William and Kate's next baby being a girl called Princess Alice.
Online bookies Betfair Sportsbook cut their odds on the name Alice from 8/1 to 3/1 following a rush of bets in the last 24 hours.
Alice is a traditional British royal name and is the middle name of Princess Anne and Prince Edward's daughter Lady Louise.
But a senior British royal has not been given this name since Queen Victoria chose it for her second daughter Princess Alice.
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The birth of Prince George's sibling is scheduled for the Lindo Wing of St. Mary's Hospital in central London in about a month's time, as was the case when he was born in July 2013.
Bookmakers find that backing for a boy and girl this time has been fairly evenly split but a huge 85 per cent of people think the baby will be blonde.
Prince George, now 20-months-old, has golden locks just like his dad had at that age, despite Kate being a brunette.
"Elizabeth had been the clear favourite until an unexpected rush of support for Alice over the last 24 hours and we've been forced to slash the price from 8/1 to 3/1."
Kate will carry out her last official engagement this Friday before going on maternity leave.
She and husband William will spend the day in south London at a number of charity events.
She had recently said that her due date is sometime in April-end, which had sparked speculation of a possible royal birth clash with the May 7 general election.