The charities said the merger would help sustain their services and increase their ability to influence public debate.
"This merger will be key to ensuring that the rights of our clients, who are heavily reliant on social care and support, are safeguarded despite widely expected cuts in public expenditure in the months ahead," said Jonathan Senker, chief executive of Advocacy Partners.
Senker has been appointed chief executive of the merged organisation, which will be called Advocacy Partners Speaking Up until a new name is chosen in consultation with staff and beneficiaries.
The organisations said they were not expecting any redundancies or service closures because they covered different locations.
Craig Dearden-Phillips, chief executive of Speaking Up, will stand for election as chair of the merged organisation.
"With the missions of the two organisations being almost indistinguishable, the merger will create a stable and long-term platform from which we aim to greatly enhance our offering to clients, deliver on our joint strategy and reach a wider public," he said.
Merger talks between the two charities began in October 2008.
Read Dearden-Phillips's record of the key events leading up to merger.