Hopes are rising that political leaders will agree on Monday to pledge enough cash to save the lives of millions of children in the world's poorest countries.

Business chief and philanthropist Bill Gates and Prime Minister David Cameron are to lead the Global Alliance for Vaccines and Immunisation (Gavi) conference in London, which will see political leaders discuss how to generate sufficient funds to ensure children 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.

International Development Secretary Andrew Mitchell said Britain would show leadership at the conference, where Mr Cameron is expected to pledge an increase in Britain's contribution to Gavi.

Mr Mitchell told BBC1's Andrew Marr Show on Sunday: "We had a look when we came into Government at all the different ways that Britain does development with British taxpayer funds and one of the very best was the Global Alliance of Vaccines and Immunisation, where effectively you can vaccinate a kid in the poor world for the price of a cup of coffee against all five of the killer diseases which mean so many of these children die before the age of five.

"We are hoping by tomorrow lunch time to have raised sufficient funding over the next four years to vaccinate 250,000 children in the poor world and save millions of lives. It's really important. It's Britain's big ask for development this year. We want to support it very strongly. We have a leadership role in all of this and as a result of the action that will be taken tomorrow, we have a real chance of saving more than four million children's lives."

Mr Mitchell also defended Britain's aid budget, which despite cuts across Whitehall has increased, prompting vocal criticism from the Tory backbenches.

He said it was not only "morally right" but in the UK's national interests to continue to fund development projects around the world, adding: "We don't protect our security only by tanks and guns but also by training the police in Afghanistan, getting girls into school in the Horn of Africa and building up government structures in the Middle East."

Mr Gates was in optimistic mood ahead of the conference. In an open telephone call, hosted by broadcaster and Save the Children ambassador Natasha Kaplinksy, he said a malaria vaccine could be just a few years away, while polio could follow smallpox in being eradicated thanks to the success of its vaccine.

Vaccines were "magic", he said. "They are very inexpensive, they can protect you for your entire life, so diseases like smallpox that used to kill millions are completely gone because of the vaccine. It's the greatest thing that ever happened in human health. We need to get them out to people and invent some more."