Freya Murray has been given the go-ahead to take the marathon place given up by injured Paula Radcliffe.
The British Olympic Association confirmed the 28-year-old Scottish athlete had been approved as a late call-up by the International Olympic Committee and athletics' world governing body the IAAF.
Marathon world record holder Radcliffe withdrew from the Olympic Games on Sunday due to a foot injury, creating space in the squad for Newcastle-based structural engineer Murray ahead of Sunday's race.
Murray had been named as a reserve for the marathon in April and has kept up her training, and said: "I am absolutely delighted to have been selected to Team GB and I will be aiming to do Britain proud in London on Sunday.
"My thoughts go out to Paula, who has been extremely supportive towards me throughout this process. Both she and UKA have been keeping me informed of the situation, so I knew there was a chance I would be competing which has helped."
Team GB chef de mission Andy Hunt believes Murray will justify her selection.
Hunt said: "We are proud to welcome Freya to Team GB. We know she has been training hard and preparing, and will arrive fully ready to compete and deliver her personal best."