There have been calls since the 1980s to cut the number of MPs sitting in the House of Commons but only now - as a marathon debate enters its final stages in the House of Lords - could it possibly be on the brink of becoming law.

The Parliamentary Voting System and Constituencies Bill is meant to pave the way for a referendum on 5 May about changing the way MPs are elected.

But its passage has been held up by objections from peers - mainly Labour ones - to plans contained in it to redraw the electoral map of Britain.

If the bill does get Royal Assent, the number of MPs will be cut from 650 to 600 and the electorate in almost all seats will have to be within 5% of 76,000 voters.

This means each constituency - with the exception of Shetland and Orkney, the Isle of Wight, the Western Isles and the geographically massive Highlands seat of Ross, Skye and Lochaber- will have between 72,200 and 79,800 voters.

Those in favour of change argue the number of MPs in the UK, per head of population, is high by international standards.

In 2009, then Opposition leader David Cameron claimed £12m a year could be saved in salary and pension costs with a 10% cut, reducing the number of MPs to 585.

'Fairer' elections

He made the comments at the height of the expenses scandal - when some members of the public would probably have happily seen the number of MPs reduced to zero.

A cull of MPs was seen by some as a way of cleaning up Parliament.

There were far too many time-servers on the Commons benches, it was argued, MPs who had long since abandoned any hope of a meaningful political career and were now simply in it for what they could get.

BIGGEST ELECTORATES 1. Isle of Wight (Con) - 109,902

2. East Ham (Lab) - 90,674

3.Manchester Central (Lab) - 90,110

4. North West Cambridgeshire (Con) - 88,851

5. Ilford South (Lab) - 87,765

What was needed, it was argued, was fewer, better motivated MPs, with a more professional outlook.

The Conservatives and Lib Dems were also keen to redress what they see as a pro-Labour advantage arising from the existing constituency boundaries. They wanted "fairer" elections.

But Labour smelled a rat. They believed the Conservatives would gain an unfair advantage at the next general election if constituencies were equalised.

Labour MPs tend to represent more of the urban or inner-city constituencies which have fewer voters. Increasing the size of the electorate in these seats would mean there being fewer of them overall.

Some in the Labour Party fear boundary changes could cost them up to 20 seats - or that it could even result in the party being shut out of power at Westminster for good.

'Frankenstein constituencies'

Critics of the government's plans also worried that the proposed changes would damage the traditional link between an MP and their constituents, with a higher workload for MPs and "Frankenstein constituencies" stitched together from different communities with little in common.

SMALLEST ELECTORATES 1. Na h-Eileanan an Iar (SNP) - 21,780

2. Orkney and Shetland (LD) - 33,085

3. Arfon (PC) - 41,198

4. Aberconwy (Con) - 44,593

5. Dwyfor Meirionnydd (PC) - 45,364

Some Labour figures also dispute whether a cut in MP numbers would actually save money once the cost of the frequent boundary reviews needed to keep constituencies the same size was taken into account.

The coalition would reduce costs a little by abolishing the right to a public inquiry over boundary changes - but Labour are also strongly opposed to this idea, arguing it would rob local people of a say.

The House of Commons has grown in size with Britain's population - from 625 MPs in 1950 to the current level of 650 by 1983, since when it has remained steady.

At the same time, the average number of registered voters living in UK constituencies has grown rapidly - from fewer than 55,000 in 1950 to 70,000 now.

But there are still those who claim British citizens are over-represented.

Only Italy, with 630 MPs in its main elected chamber comes close among major Western countries.

The UK has one elected representative for every 91,000 citizens, according to research by Robert A Dahl, Sterling Professor emeritus of political science at Yale University. The US, by comparison, has one elected representative for every 673,000 citizens.

Lib Dem damage

POPULATION PER ELECTED REPRESENTATIVE 1. UK - 91,500

2. Italy - 92,200

3. France - 104,700

4. Canada - 105,500

5. Australia - 132,700

Source: R Dahl On Political Equality (Yale University Press)

The traditional assumption is that redrawing constituency boundaries, so that they are all roughly the same size, and cutting the number of MPs in the Commons will favour the Conservatives.

But research by Democratic Audit for BBC Two's Newsnight suggests it is the Liberal Democrats who would have lost out most if the proposed changes had been implemented at the last election.

The party would have lost 12% of seats - seven of 57. Labour would have lost 10% - 25 of 258, and the Tories 4% - 13 of 307, the estimates show.

Part of the reason for this, according to researchers, is that Lib Dem seats tend to have smaller majorities and do not clump together as much as those of the other parties, and boundary changes "mean that areas which are not necessarily unsympathetic to the party, but where the party has not previously campaigned hard, are added from neighbouring constituencies".

But with so many variables, even the experts admit it is impossible to accurately predict what the impact would be - particularly when no-one knows at this stage where the new boundary lines would be drawn.

That is the key factor - and the source, no doubt, of much controversy around the country if and when the bill becomes law.

Here is a selection of your comments:

Cut the number of MPs now, after all we already pay additional for MEPs, MSPs, etc. Labour is being regressive in deliberate delay tactics, looks like they just want to keep "jobs for the boys" Lixxie, Glasgow

having just spent the last year defeating the BNP and the mess left behind by the insanity of the established elitest Oxbridge set who represent themselves and their own financial interests above and beyond anything else I see no problem in seeing as a big a reduction as possible. Only the Cabinet have any real power and scrutiny is limited to the range of their own shared self-interest as they dish out Governmnet contracts with our money. They have made themselves redundent. Ralph, London

Professor Dahl's figures are misleading because they do not take account of the fact that Canada, Australia and the USA have powerful sub-national levels of government - provinces, territories and states - which also have elected legislatures. The UK does not have the same system; the devolved authorities we have are much less powerful. Take that into account and the UK has the fewest elected representatives per head of population in any major Western democracy. David, United Kingdom

Surely making the vast majority of consituencies represent the same amount of voters (give or take 5%) is more democratic and representative of what voters want than the current system. I cannot understand how this would give the conservatives and 'unfair' advantage and can only conclude that currently Labour are the ones with the unfair advantage through representing smaller constituencies. One of the reasons I support the coalition is this attempt to rebalance the system. Laura, Solihull

It seems to me as if standardising population size per constituency is a great idea - HOWEVER, I think either all the parties should equally collaborate on this, or else get someone independent to do it. Nothing "Frankenstein-ises" a constituency more than changing boundaries every time we change government. The boundaries should be fixed, and not changed unless there is a population change. Cutting MPs I wholeheartedly agree with - I even think it might be worthwhile doing a bit more. Next cuts... the Lords, please. Anon

The issue of which party will or won't suffer the most is irrelevent if you believe that our parliament should be democratic, i.e. you believe that everyone having an equal say in how parliament is formed is more important than making sure your prefered party comes out on top. This leaves the question of whether boundary reform makes the situation more or less democratic. Unequal constituencies (and a non-PR based voting system) mean that the weight of each constituents vote varies dependent on the size of the constiuency they belong to; if you live in a constituency of larger numbers, your vote has less influence on the outcome of a general election than someone from a smaller constituency. There are many other arguments for and against equalisation, but from a purely democratic point of view making each vote have closer to an equal amount of influence on parliament would appear to move the voting set-up towards a more balanced democracy rather than the other way, regardless of which party it hurts, it would appear to allow parliament to more accurately represent the political views of the public. An interesting point to consider though... are individual opinions more or less important than the collective views of each community? Alex, Cambridge

I'm all up for a reduction in MP's but since we have separate goverments for scotland, wales etc... we either need to move towards having a English only Parliment or at the very least only english MP's voting on English only related issues! Peter, Oldham

There have been many separate boundary changes designed to address the imbalance in constituency size and allow for the movement of people from town to country and vise versa. Labour's objection to giving advantage to the Tories is ridiculous, boundaries should be equal to the best of ability, the system should favour no-party, and quite clearly the system currently favours Labour. It is a huge hypocrisy to apparently support a referendum on AV while objecting to equal constituency sizes where the relative value of a votebetween constituencies is different. George, Southampton