By Joseph Lee
BBC News
The 95-year-old monarch underwent preliminary medical checks in hospital last Wednesday after cancelling a visit to Northern Ireland.
She resumed public engagements on Tuesday by meeting ambassadors via video link from Windsor Castle.
Buckingham Palace said she “regretfully” decided not to attend a reception at the summit.
But the palace said she would deliver her address to delegates using a recorded video message instead.
It is understood that the Queen very much wants COP26 to result in meaningful action on climate change from participating nations, and hopes her absence will not be used by others as a reason not to attend.
In photographs released on Tuesday, the Queen was seen smiling on camera as she greeted the South Korean and Swiss ambassadors, who were speaking to her from Buckingham Palace.
It was the first time she had been seen since she hosted an investment summit at Windsor Castle on the evening of 19 October.
The following day, Buckingham Palace said the monarch had “reluctantly accepted medical advice to rest for the next few days”.
A decision not taken lightly
A cancelled trip to Northern Ireland and a night in hospital last week were followed by reassurances that this would only mean a rest and recharging of the royal batteries ahead of the COP26 summit.
That trip to Glasgow has now been cancelled too. It was a big moment in the royal calendar and it’s a decision that would not have been taken lightly.
Her meetings on Tuesday were held on video – and a video message will be sent to the Glasgow summit – so perhaps this will be more of how we’ll see the monarch in future.
She will be more online, while those in-line will take up more of the public responsibilities.
She spent the night of 20 October in hospital before returning the next day to Windsor Castle, where she was said to be “in good spirits”.
In its latest statement, the palace said: “Following advice to rest, The Queen has been undertaking light duties at Windsor Castle.
“Her Majesty has regretfully decided that she will no longer travel to Glasgow to attend the evening reception of COP26 on Monday, 1 November.”
The COP26 global climate summit in Glasgow in November is seen as crucial if climate change is to be brought under control. Almost 200 countries are being asked for their plans to cut emissions, and it could lead to major changes to our everyday lives.
The statement concluded: “Her Majesty is disappointed not to attend the reception but will deliver an address to the assembled delegates via a recorded video message.”