The UK must introduce safeguards to ensure governments cannot "pick and choose" how to bring in constitutional change, peers say.
The Lords constitution committee says ministers should explain to Parliament the thinking behind bills and the way in which policies are developed.
Its chairman, Labour's Baroness Jay, said there was often "insufficient regard" for the public good.
Lawmaking must be a "clear and consistent" process, she added.
In its report, published following a six-month inquiry, the committee argues it is not acceptable that the UK has no agreed process for constitutional change.
It adds that constitutional legislation should meet the highest standards of consultation, consideration and scrutiny.
The report sets out a "comprehensive package" of measures to follow, including publishing consultation documents and draft bills.
The committee also recommends that the minister responsible for initiating constitutional change should explain to Parliament the processes used to develop the proposals when a new bill is introduced.
Baroness Jay, a former House of Lords leader, said: "Currently, there is little to stop governments with a Commons majority from changing the UK's constitutional arrangements as they please.
"The constitution is vulnerable to the political agendas of successive governments, with insufficient regard to what is best for the people. This is unacceptable.
"We must make sure that this country's constitutional framework is respected and treated with more care by those who seek to change it. This can only be done by introducing a rigorous process for bringing about such change.
"My committee urges the government to take up its recommendation as to what that process should be."
In 2009, the then Prime Minister Gordon Brown asked Cabinet Secretary Sir Gus O'Donnell to head up efforts to "consolidate the existing unwritten, piecemeal conventions that govern much of the way central government operates under our existing constitution into a single written document".
But, in March, the committee said the resulting document, the Cabinet Manual, was not a "first step" to a written constitution.
It should serve as a point of reference for civil servants but not "set in stone" the processes by which ministers reach decisions, it added.