Rogue charity fundraisers taking advantage of vulnerable people's generosity will face a new law aimed at stopping their "unacceptable" behaviour, David Cameron said.
The Prime Minister 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.
The Prime Minister said: "Our charities undertake vital work, bringing communities together and providing support to some of the most vulnerable members of our society.
"But the conduct of some fundraisers used by them is frankly unacceptable and damages the reputation of the sector as a whole, which is why we're introducing a new law to make sure charities raise funds in the right sort of way."
Under the new rules, charities with incomes over £1 million will set out in their trustees' annual report their fundraising approach, including the use of professional fundraising agencies – as well as steps to prevent inappropriate fundraising from vulnerable people.
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.
The fundraising methods of major charities have come under the spotlight since the death of Bristol poppy seller Olive Cooke in May. At one point the 92-year-old received 267 charity letters in one month, leading to suggestions that the hounding for money pushed her to take her own life.
Her family insisted that - while the letters and phone calls were intrusive and a nuisance - the charities were not to blame for Mrs Cooke's death.
In separate allegations, four of the country's biggest charities are being investigated amid claims fundraisers contacted people on the Government's "opt-out" database.
The NSPCC, British Red Cross, Oxfam and Macmillan Cancer Support have been accused of making fundraising calls to people registered with the Telephone Preference Service (TPS), which protects them from receiving unsolicited sales or marketing calls.
The accusations have been made after an undercover reporter with the Daily Mail worked in a call centre fundraising on behalf of the charities.
Minister for Civil Society Rob Wilson said: "After the tragic death of Olive Cooke, I made clear that the behaviour of charities had to change or we would take action - today I am delivering on that promise.
"This law will force charities to face up to their responsibilities."
Alongside the changes, the chief executive of the National Council for Voluntary Organisations (NCVO) Sir Stuart Etherington will chair an urgent review to consider whether further regulatory powers are required.
Sir Stuart said: "Britain's charities are the backbone of our society. For every challenge we face, there is a charity that will help – be that facing cancer or saving our precious green spaces. They are able to do this because of the trust, confidence and overwhelming generosity of the British people.
"We simply cannot allow aggressive fundraising tactics to damage this trust. The public demands a simple and effective system of regulation. My review will respond to clear public concern about fundraising that has overstepped the mark. Charities must maintain the highest standards, including taking a firm grip on how the agencies they employ are interacting with the public."