The Queen is undertaking her first public engagement outside a royal residence since March, as she is joined by the Duke of Cambridge for a rare joint outing to Porton Down.
The Queen and Duke are visiting the Defence Science and Technology Laboratory (Dstl), near Salisbury, where they will see displays of weaponry and tactics used in counter intelligence before meeting those involved in the Novichok poisoning of 2018.
She will formally open the new Energetics Analysis Centre, while the Duke presents an award to members of the military who helped with the Novichok clean-up.
Heightened safety precautions were taken ahead of the visit to protect the Queen against coronavirus.
All 48 people who were due to come into close contact with the monarch and the duke were tested for Covid-19 by Dstl. Small groups of those taking part in the royal visit were arranged two metres apart for social distancing.
The Queen also arrived by helicopter, separately from William. The pair last saw each other at Balmoral over the summer.
A Buckingham Palace spokeswoman said: "Specific advice has been sought from the medical household and relevant parties, and all necessary precautions taken, working closely with Dstl."
Grandmother and grandson will be greeted by chief executive Gary Aitkenhead, and will be given a private tour of the Energetics Enclosure.
They will then see a demonstration of a Forensic Explosives Investigation and meet staff employed in counter terrorism and security.
Buckingham Palace said the Queen and Duke will meet Dstl staff involved in identifying the nerve agent and subsequent clear up of the Novichok incident of 2018, as well as scientists providing vital support to the UK response of the COVID-19 pandemic.
They will hear about the deployment of expert microbiologists to NHS hospitals to increase testing capacities.
It is a rare joint engagement for the Queen and Duke, who only generally appear together at family or major ceremonial occasions.
It is the first live engagement the Queen has been able to undertake outside her own home since the outbreak of coronavirus.
Since March, she has been seen on numerous occasions on video calls and in photographs, in addition to her special television addresses.
In July, she performed a special outdoors ceremony to knight 100-year-old Sir Tom Moore in the grounds of Windsor Castle.