As London Mayor Boris Johnson launches his official campaign for re-election, London homelessness charities believe he has short-changed them to the tune of more than £5m.

Charities are concerned that £5.3m of the £33.8m budget given to the Mayor by central government to address homelessness and rough sleeping in the capital in the four years to 2015 has not materialised in budget planning.

The money was not formally ring-fenced, and according to published budgets, the Mayor underspent in the first year, spending £7.5m as opposed to £8.5m. Current plans have the Mayor spending an additional £21m over the following three years, which brings the total to £28.5m – significantly short of the original allocation.

A joint letter to the Mayor, signed by both national and London-based homelessness charities, has called for the return of the apparent shortfall in homelessness service spending, particularly at a time of reported increase in demand on charities working in the capital.

A spokesman for Johnson’s re-election campaign told the Guardian that the funding already allocated will enable the extension of the ‘No Second Night Out’ scheme across all of London.

“Since 2008, there has been a three-quarters reduction in the most entrenched rough sleepers and 80 per cent of new rough sleepers just spend one night on the streets. We cannot go back to the waste and neglect of the previous administration and Boris will build on this strong foundation in a second term,” the spokesman was quoted as saying.

In February 2009, the Mayor set up a London Delivery Board to end rough sleeping in London by the end of 2012.