The ruling coalition faces its fiercest internal battle yet over arcane plans to give people a vote for the upper chamber of parliament, the House of Lords, an issue that has vexed politicians for 100 years but leaves most voters unmoved.

Prime Minister David Cameron and his deputy, Liberal Democrat leader Nick Clegg, say reform of the roughly 800-seat upper chamber, which traces its roots back a millennium to Anglo-Saxon times, will strengthen British democracy.

But Cameron faces stiff resistance from within his own Conservative ranks that could weaken him and sour relations with Clegg, whose party is strongly in favour of move. The row has the potential to split or paralyse the coalition for a time, although few expect it to bring down the government altogether.

Polls suggest that most voters are broadly in favour of an elected upper house, as opposed to the present chamber largely appointed by party leaders according to their strength in the lower house. But it is very low on their priority list.

Yet for Lib Dems, who have seen their support collapse and some of their cherished goals elude them since they entered government for the first time in 2010, the Lords is profoundly undemocratic and reforming it is an article of faith.

Disavowed by many who voted for them two years ago only to see the junior party buckle to Conservative demands, Lib Dems see this as their chance to leave a lasting legacy.

I just feel it's an absurd anachronism. I feel embarrassed when we in the Lords, after a good debate perhaps, can't carry the day because we have no mandate, we have no legitimacy, Paul Tyler, a Lib Dem who sits in the upper house with the title Lord Tyler of Linkinhorne, told Reuters on Monday.

During debates in their ornate red and gold chamber in the Palace of Westminster, peers address each other as the noble lord, the noble baroness or, if they are from the same party, my noble friend. Protocol requires that peers who are also bishops be addressed as noble and right reverend lord while some senior military commanders are noble and gallant.

Such archaic conventions date back to past centuries when the chamber was made up of Lords Spiritual - bishops of the Church of England - and Lords Temporal, or aristocrats. They project a fusty image to many and today most Britons are barely aware of the work of the Lords.

But behind the pomp and ceremony a debate is raging.

DUELING CHAMBERS

Conservative rebels, including some cabinet ministers, say parliament will be bogged down for months in recondite arguments over constitutional arrangements when it should be spending its time on more urgent problems such as fixing the economy.

The Lords' power was largely removed in favour of the elected lower chamber, the House of Commons, in 1911 in a reform led by the Lib Dems' political forebears, the Liberals.

The peers in the Lords complement the work of the MPs in the Commons in making laws and holding the executive to account. But though they may revise legislation - and indeed are prized by many for their expertise - the Commons has the final say.

Opponents of the new reform plan say that electing the Lords would enhance the power of Britain's disciplined and powerful party machines, making the chamber less consensual and more partisan as well as more assertive, potentially challenging the primacy of the Commons and leading to parliamentary deadlock.

We are in danger of seeing duelling chambers flaunting their rival democratic mandates, said Peter Hennessy, a journalist and historian who entered the Lords in 2010 as Baron Hennessy of Nympsfield. A crossbench peer, he has no party.

Matters are coming to a head because the government intends to put forward a bill on reform as part of the May 9 Queen's Speech, the annual ceremonial presentation by the monarch of the government's legislative programme for the coming year.

The aim is to pass the bill as quickly as possible so that the first elections to a reformed House of Lords can take place in 2015, when a general election to the Commons is also likely.

After months dissecting the government's draft plans for Lords reform, a joint committee of MPs and peers from all major parties published their recommendations on Monday.

The main ones were that 80 percent of peers should be elected, while 20 percent should be appointed and that the size of the House should be cut from over 800 to 450 members, who would each serve for one 15-year term.

But the committee chairman Ivor Richard, or Baron Richard of Ammanford, from the Labour party said it had been impossible for the 26 members to agree on some key elements of the proposed bill. This does not bode well for its passage through the 650-seat Commons and the even bigger Lords.

Describing the subject matter as dense, Richard, a former MP and European Union commissioner, said he hoped that, should there be a need for another committee on Lords reform in future, someone else would chair it.

After his official presentation, rival factions staged briefings in different corners of the labyrinthine Palace of Westminster, home to both houses of parliament on the banks of the Thames. As they set out their competing views, it offered a glimpse of the complex political manoeuvres lying ahead.

INDUSTRIAL-SCALE PATRONAGE

As things stand, Lords serve for life and most are appointed by parties, under a system Clegg has described as industrial-scale political patronage. A seat in the Lords has long been seen as a reward for major party donors, retiring MPs or others with friends in high places.

The average age of peers is 69 and there are more who are over 90 years old than under 40. Only one in five is a woman and a disproportionately high number come from the affluent southeast of England.

The last major Lords reform, in 1999 under the Labour government of Tony Blair, got rid of hundreds of hereditary peers, scions of the old aristocracy, leaving a rump of 90 of them. The number of appointed peers has soared since then.

Blair created 162 Labour peers during his decade in office, while in two years as prime minister Cameron has already appointed 47 Conservatives to the upper house. At each occasion, prime ministers also appoint peers from other parties. Tyler said that if the system was left unchanged, it was only a matter of time before the number of Lords grew to over 1,000.

He said there was a lot of self-interest in the anti-reform camp, not only among Lords who were reluctant to vote themselves into extinction but also among MPs with their eye on a seat in the Lords later on in their careers.

But his opponents rejected that accusation, saying that while there was broad cross-party agreement on the need to reform the Lords, the current proposals were half-baked.

Said Conservative MP Nadhim Zahawi: The nation won't thank us for an ill thought-out reform.

(Editing by Alastair Macdonald)