There were fears for the Queen’s health the week before last when she pulled out of a service at St Paul’s Cathedral 'feeling under the weather.'
But she recovered to spend a full week at Holyroodhouse in Edinburgh, her official Scottish palace.
An insider said: 'This was not a last minute decision. It was agreed sometime ago that the Queen and the Duke would not be at the christening.
'Everyone is relaxed about it and it was felt it was the right decision for all concerned.
'The fact is that the Queen has another busy week ahead.
'She has just come back from Scotland, will be at the Palace for an RAF flypast tomorrow (tuesday) and meets Donald Trump later in the week.
'It is fair to say that she is pacing herself.'
Royal author Phil Dampier said: 'Reality is catching up with the Queen.
'The Duke is 97 and retired last year. The Queen is 92 and can’t do everything. I think this is very wise as her health must come first.'
Dampier adds, 'The UK is in the middle of a heatwave and she has possibly been overdoing it recently.
'There was a report her knees are playing up and she must be careful.
'It is only sensible that she misses out on some events although I’m sure she will be upset to miss the christening.
'She is devoted to her children, grandkids and seven great grandchildren. She likes to treat them all equally, but I’m sure William and Kate totally agree with this decision.'
Earlier, the Palace named Louis’s six godparents, which you can see here