Rogue charity fundraisers who take advantage of vulnerable people's good will are to face a new law aimed at stopping their "unacceptable" behaviour, the prime minister has said.

David Cameron said the actions of aggressive fundraisers and the organisations employing them was damaging the reputation of the entire charity sector.

The new law will force charities and fundraisers to have a written agreement showing how the vulnerable will be protected. Some of the UK's biggest charities will also be forced to reveal their fundraising methods to show they are behaving properly.

David Cameron said some tactics used by fundraisers are 'frankly unacceptable'. Credit: PA

Our charities undertake vital work, bringing communities together and providing support to some of the most vulnerable members of our society.

David Cameron

The legislation will also require all professional fundraisers to set out in their agreements with charities what steps they are taking to protect vulnerable people from high pressure tactics and how the charities will monitor compliance.

The changes will be introduced in amendments to the Charities Bill currently going through Parliament.