The government has published a draft "Cabinet Manual" describing the workings of the UK's unwritten constitution for the first time.
The 11-chapter document, produced by civil service head Sir Gus O'Donnell, deals with the roles of the monarch, the government and Parliament.
It also looks at relations with the European Union and devolved regimes in Scotland, Wales and Northern Ireland.
MPs and peers are to decide on the document's final draft.
Unlike some countries, such as the United States, the UK has no written constitution, instead relying on accumulated statutes, conventions and rulings.
The Liberal Democrats have long campaigned for this to change, with previous Labour Prime Minister Gordon Brown - who commissioned the manual earlier this year - also hinting that this might be possible.
'Not set in stone'
But the coalition agreement between the Lib Dems and Conservatives does not commit the government to setting down the constitution on paper.
Sir Gus emphasised that the manual was not a constitution and had no legal force, but would serve instead as a guide.
In his introduction to the first draft, Sir Gus wrote: "The Cabinet Manual is intended to be a source of information on the UK's laws, conventions and rules, including those of a constitutional nature, that affect the operation and procedures of government.
"It is written from the perspective of the executive branch of government. It is not intended to have any legal effect or set issues in stone. It is intended to guide, not to direct."
He added: "Some areas of the manual continue to be subject to public debate. The manual, however, does not seek to resolve or move forward those debates, but is instead a factual description of the situation today. In other words, it will be a record of incremental changes rather than a driver of change."
The manual covers statutes dealing with constitutional matters dating back to Magna Carta in 1215.
Its first sentence reads: "The UK is a parliamentary democracy which has a constitutional sovereign as head of state; a sovereign Parliament, which is supreme to all other government institutions, consisting of the sovereign, the House of Commons and the House of Lords, an executive drawn from and accountable to Parliament, and an independent judiciary."
Sir Gus said it would be "inappropriate" for the manual to anticipate constitutional reforms proposed by the government, such as fixed-term parliaments, a reduction in the number of MPs, a referendum on the alternative vote for Westminster elections, House of Lords reform and referenda on EU treaties.
He invited interested parties to comment on the first draft by March before the publication of the final version of the manual is published next year, subject to approval by Parliament.
Peter Riddell, senior fellow of the Institute for Government, said: "Merely by writing down the official view of current practice, the draft may be seen, wrongly, as a step towards a written constitution. It is not.
"Its authors expect, or perhaps hope, that the draft will not be used by the courts, and nothing in the report looks obviously justiciable. A written or codified constitution raises far wider questions about the role of judges which are not remotely near being addressed, let alone resolved."