A joint letter from the two organisations to representatives of the Conservative, Labour, Liberal Democrat, Green and UK Independence parties asks for whoever is the next Economic Secretary to the Treasury to commit to ensuring that "no steps are taken that would reduce the amount of eligible Gift Aid claims".
It also says that the minister should work to make increasing the amount of Gift Aid claimed a priority.
The letter says that Gift Aid is not claimed on £2.3bn of donations a year and, although not all of these would be eligible, "there is potentially hundreds of millions of pounds of Gift Aid going unclaimed".
It says: "This is money that should be furthering the work of charities, helping them to meet growing demand and deliver public benefit."
It says that since it was reformed in 2000, Gift Aid has grown substantially but in recent years that growth has slowed.
"While a slowdown was inevitable, it is even more important that, as Gift Aid matures, we redouble our efforts to improve take-up," it says. "We recognise that there are a number of pressures on public finances and that there might be a temptation to focus on reducing the ‘tax gap’ rather than increasing eligible donations. However, we believe that the next government must consider this in a holistic way."
The letter suggests commissioning more research into Gift Aid and the factors involved in claiming, using government-backed campaigns such as Giving Tuesday to promote Gift Aid and working with charities to improve public awareness and understanding of the scheme.