Dame Sarah Storey has withdrawn from the Great Britain team for the Para-cycling Track World Championships in Rio de Janeiro.
The 14-time Paralympic champion announced her decision following advice regarding yellow fever vaccination after an outbreak of the disease.
Storey gave birth to son Charlie last year and was due to make her return to competitive cycling in Brazil but as he is breastfed, neither mother nor baby can be vaccinated, prompting Storey to reluctantly pull out.
"I have made the decision to withdraw from the UCI Para-cycling Track World Championships after the advice regarding the Yellow Fever vaccination changed last week," Storey said in a statement.
"I am obviously disappointed to not be able to compete at the event and miss out on the opportunity to win another rainbow jersey but the health and well-being of my family is far more important.
"Ordinarily this wouldn't have been a problem but unfortunately there are risks around the Yellow Fever vaccination in certain groups including breastfeeding women and babies under the age of nine months.
"As Charlie is still only four and a half months and breastfed this means neither of us can be vaccinated and therefore I am not prepared to risk travelling as the infection results in a high percentage of fatalities.
"My focus will now turn to the UCI Para-cycling Road World Championships in Italy in August and I'll be working with my team Storey Racing to start my road programme earlier than expected.
"The timing of this couldn't have been predicted and I would like to thank all the staff on the Great Britain Cycling Team who have been fully supportive of my decision."