By Bob Howard
BBC Radio 4's Money Box
Charities are concerned that changes in taxation and life expectancy will have a significant impact on their funding.
In his last budget as Chancellor, Gordon Brown revealed the tax cut
In April the basic rate of tax will fall by 2 per cent, impacting on "gift aid", which allows charities to reclaim donors' income tax from the Revenue.
The Institute of Fundraising says this will reduce charity income next year by about seventy million pounds.
Charities hope that more people will leave them a gift of money in their will, to make up for the shortfall.
Life and taxes
Married couples and people in civil partnerships will benefit from the doubling of the individual tax free IHT allowance, according to the chancellor's pre-Budget report.
Fundraisers are worried that these changes will deter those donors who leave legacies to charity to avoid paying money to the government.
Changes to life expectancy are also impacting on the number of legacies charities receive each year, says Duncan Bell, head of legacy fundraising at Cancer Research UK:
"Over the next decade or so, fewer people will die in the UK year on year.
"That is a demographic fact, and we have to deal with that."
How to gift
You can leave a gift of money as a lump sum, or as a share of your estate - both have pros and cons:
A lump sum is simple, but a figure that sounds impressive today may have depreciated greatly decades later.
A share of your estate is likely to be more generous, but if house values and other assets rocket, you may donate a lot more than you intended, or if they go down, you may leave less.
Also, giving a charity a percentage of what you leave after you die is much more complex than a lump sum, for whoever has to sort out your estate.
Cynthia, from Farnham in Surrey, wants to leave most of her estate to her two daughters and three grandchildren, but she also wants to leave a legacy to Cancer Research UK, after she successfully beat the disease.
"It seemed a better idea to leave a lump sum - I will upgrade my will so it is not something that is going to remain static."
Current and correct
Legal experts agree that regularly reviewing your will is important, if you want your beneficiaries to receive what you intend them to.
Christine Haniver is head of probate at Cornfield Law in Eastbourne:
"People like to do their will once and that is the end of it.
"I always suggest clients look at it every three to five years."
Sometimes people leaving a legacy overlook simple things, like getting the name of the charity precisely right.
Cancer Research UK's Duncan Bell says that can cause no end of trouble:
"There is a problem with people leaving money to 'Cancer Research' which does not exist."
"Unless there is another way of identifying us, such as our registered charity number, or our address, then the money is divided between a number of charities with 'cancer research' in their title."
BBC Radio 4's Money Box was broadcast on Saturday, 01 December 2007 at 1204 GMT.