Business chief and philanthropist Bill Gates is to take part in a telephone conversation with British people today ahead of a global conference on vaccines.
Mr Gates will participate in the event the day before he and Prime Minister David Cameron lead the vaccine summit in London.
The Global Alliance for Vaccines and Immunisation (GAVI) conference will see political leaders discuss how to generate sufficient funds to ensure children in the world's poorest countries receive the vaccines they need.
GAVI is facing a shortfall of £2.3 billion for its work over the next five years, charities have warned.
Britons were asked to submit questions for Mr Gates on why so many of the poorest children still die because they cannot get the life-saving vaccines taken for granted in rich countries - and what needs to be done to save their lives.
They can join Mr Gates in the call at 11.30am to hear some of their questions answered.
Broadcaster and Save the Children ambassador Natasha Kaplinksy will host it, and Justin Forsyth, the charity's chief executive, will also take questions.
Ms Kaplinsky said: "This is an amazing opportunity for people in the UK to ask Bill Gates about saving children's lives, just before he heads into one of the most important global meetings of this decade.
"World leaders have an incredible opportunity to save four million children's lives in just four hours, and the British public can be part of this historic breakthrough."
GAVI's global immunisation programme includes the roll out of new vaccines against two of the biggest child killers, pneumonia and diarrhoea.
Save the Children's new report 'Vaccines for All' reveals that there are still around 24 million children - the world's poorest, and those most vulnerable to disease - that have no access to vaccines.
This could be changed if GAVI gets the resources it needs, it says.
:: To put a question to Mr Gates, or join the call today, go to: http://www.savethechildren.org.uk/billgates