UK charities' fundraising activities could be controlled by law unless a new voluntary regulator succeeds in cleaning up the sector, a committee of MPs has warned in a report.
They said it was the "last chance" for self-regulation of charity fundraising.
The new regulator is being set up following scandals last summer, when unscrupulous fundraisers were accused of targeting old and vulnerable givers.
MPs said most charities did not engage in such practices.
But the behaviour of some had damaged the reputation of all, they said.
MPs on the Public Administration and Constitutional Affairs Committee (PACAC) heard that some charities, including Great Ormond Street Hospital and Macmillan Cancer Support, made it impossible for donors to block further communication from them or other charities.
Vulnerable and elderly people were seen as "fair targets" by some organisations, and information sold by some charities ended up in the hands of scammers, MPs were told.
Victims of aggressive charity fundraising
Fundraising practices came under scrutiny in 2015 after the death of 92-year-old Olive Cooke, one of Britain's longest-serving poppy sellers.
She killed herself after complaining she had been plagued by requests for donations from charity fundraisers - more than 200 letters in one month alone.
While her family said that did not contribute to her death, it did highlight the plight of vulnerable people at the mercy of unscrupulous fundraisers.
Samuel Rae, 88 - a former army colonel with dementia - was another victim.
Charity fundraising companies sold his details on to other companies, including fraudsters who targeted him for £35,000.
Nick Booth, from London, told the BBC News website he was targeted with cold calls from a charity.
"I was phoned out the blue by what sounded like a double-glazing cold caller," he said. "She expected my bank details for a standing order. Age Concern was mentioned, but only in the monotone of someone reading out terms and conditions.
"When I said I wasn't sure about giving complete access to my bank account to a complete stranger, she said, 'Oh, so you don't like helping old people, eh?' I'm never giving my details out again."
Peter Anscombe, from Somerset, said: "As a charity trustee, a deputy chair of a charity, and as a regular donor I am dismayed at some of the heavy selling techniques across the sector.
"I have personally stopped giving to some charities as a result of their aggressive approach. The intensity of people chasing you for money just grows and grows."
The MPs said charity trustees who had permitted scandalous fundraising methods were either "incompetent or wilfully blind".
Chairman Bernard Jenkin said: "This is the last chance for the trustees of charities, who allowed this to happen, to put their house in order.
"Ultimately, the responsibility rests with them. No system of regulation can substitute for effective governance by trustees."
He told the BBC that charities were "apologetic", but there was not yet a "proper understanding... that it is fundamentally trustees who are responsible for setting the tone of their organisation".
"Their values should extend to everything they do, not just the charitable objectives," he added.
Rob Wilson, minister for civil society, said: "I have made it clear that the sector has one last chance to prove that self-regulation can work, but I am willing to step in and impose statutory regulation if necessary."
The government's response to the scandals was to set up a review which recommended a new regulator of fundraising.
Former Big Lottery Fund chief executive Stephen Dunmore was appointed in December as interim chief executive of the new regulator, responsible for setting up the organisation.
It will replace the Fundraising Standards Board (FRSB), which has welcomed the call for trustees to take more responsibility for fundraising activities.
Like the FRSB, the new body will be based on self-regulation - meaning charities themselves develop and enforce standards and rules of behaviour.
Sir Stuart Etherington, who chaired the review, said it was important not to "lose sight of the principle of self-regulation in fundraising on which everyone is agreed".
"I am not persuaded for this reason that it is appropriate for the fundraising regulator to report to a government body," he added.
Paul Farmer, chairman of the Association of Chief Executives of Voluntary Organisations, which represents charity leaders, said the new regulator had the "full support" of charities and should "make sure that the poor practices of a tiny number of charities don't happen again".
He also said charities had "looked really carefully" in recent months at their procedures on using donors' data.