Kate, who is believed to be at least three days overdue, is set to deliver her baby at the Lindo Wing of St Mary’s Hospital in Paddington.
Rumours of the presence of royal protection officers are circulating on Twitter, sparking suggestions the royal baby could be very imminent indeed.
Tick tock: Royal baby watchers at the Lindo Wing
According to Get West London, police and a sniffer dog have been spotted outside the hospital and a photographer know to be close to Kate’s PA Natasha Archer has also been seen on the premises.
But it points out the activity may simply be a “dry run”, one of which was also held ahead of the birth of Prince George in 2013.
It writes: “Officers and security staff often run through their roles and responsibilities a number of times before the event, to make sure everyone knows what they have to do.”
The delay means William and Kate's baby - fourth in line to the throne, the Queen's fifth great-grandchild and a younger brother or sister for Prince George – will not meet its Uncle Harry until he is back from Australia in mid May.
The Prince is returning overseas to spend time with Australian Army units in Perth and Sydney before he embarks on an official tour of New Zealand from May 9 to May 16.
Bookmaker Coral has odds of 5/1 that the baby will arrive on the Duke and Duchess's fourth wedding anniversary tomorrow, but said most punters believe the newest addition to the royal family will still arrive in April, rather than in May.
Coral's Nicola McGeady said: "Punters are now guessing the royal baby's month of birth rather than the date of birth after the weekend gamble folded, with the odds suggesting that Kate will give birth sooner rather than later."
The chance of the baby arriving on the same day as the General Election - May 7 - is put at 16/1.
Bookmaker Paddy Power said one customer has placed a £5,000 bet on a May Day birth at odds of 9/1 and stands to win £50,000 if the gamble proves correct and Kate delivers a son or daughter on Friday.
The Duchess had said her baby was due to arrive mid to late April.
Fewer than one in 20 women give birth on their due date. On the NHS, induction is offered to women who go two weeks overdue.