David Cameron will use a conference in London to promote plans to raise a further $3.7bn (£2.25bn) in global aid to increase immunisation programmes, further antagonising those in Britain who claim he is putting overseas aid before squeezed living standards in the UK.

With his modernising credentials damaged by the row about NHS reforms, Cameron is determined to show that he is committed to a generous UK aid budget, and reassure those on the centre left he is a centrist Conservative. He also believes he can see off the aid sceptics in his own party, mainly from the right.

In his most high-profile intervention on overseas aid since becoming prime minister, Cameron will host the Global Alliance for Vaccines and Immunisation (Gavi) conference in London on 13 June.

The conference, discussed by Barack Obama and Cameron last week, is regarded as vital to efforts to lower child mortality in Africa. In a sign of the scale of pledges being sought, the Obama administration is being asked to give $450m to the programme over three years.

Britain will also announce a substantial extra contribution to help reach the $3.7bn required to scale up immunisation programmes between 2011 and 2015, and save an estimated 4 million children's lives.

The funding will specifically enable Gavi to distribute two vaccines, pneumococcal and rotavirus, tackling the two biggest killers of children in the developing world: pneumonia and diarrhoea. It is thought the vaccines will save more than 4 million lives by 2015. Pneumonia accounts for 20% of all deaths of children under five.

Britain gave £150m to Gavi in March last year, and since 2005 has been the second most generous contributor to the alliance after the Bill and Melinda Gates Foundation.

Gavi was praised by the Department for International Development in a review on multilateral aid published three months ago. Officials at the department were reassured by steps Gavi is taking to be more transparent about the differing costs of vaccines from different producers. Two pharmaceutical industry representatives sit on the board of Gavi.

Germany announced last week that it would give €30m (£26m) to GAVI in 2012, up from €20m in 2011. Cameron made passionate remarks at his press conference at the close of the G8 summit of industrialised nations in France last week, insisting he would not backtrack on a commitment to increase the amount of overseas aid given by DfID. It is the only UK department not facing budget cuts.

Cameron added he would not solve the UK deficit on the backs of the world's poor, and lambasted some other world leaders for forgetting their promises on aid. Some of this passion was driven by the knowledge of what the Gavi conference could achieve next month. He regards practical vaccines that are shown to have tangible results in terms of saving lives as one of the best ways of combating the aid fatigue currently gripping the UK.

Gavi claims it has saved more than five million lives in its first decade of existence.

Alan Duncan, the international development minister, sprung to Cameron's defence telling Sky News that "aid-bashing does not actually get us anywhere. If we were to cancel the aid budget altogether it wouldn't solve all the other problems, so this sort of balancing of the aid budget versus all other problems isn't entirely logical.

"The fact is, if you had a pound, would you give a halfpenny to stop someone dying in the street? The answer is you probably would, and what we are doing is stopping millions of people dying from disease, we are helping educate people and make them healthy."