The Fortescue coat of arms tilts at a drunken angle over the door of Ebrington Manor. Carved in stone, it announces that this magnificent Cotswold pile, a hedge funder's wet dream, is home to the Earls Fortescue, and has been for more than 500 years. No Liz Hurleys here, thank you. But there is trouble ahead.

The current custodian, the 8th Earl, is father to three daughters, but no son. Because of the rules of succession, when he dies, the title and at least part of the estate will pass to the nearest male relative, a London-based cousin. It is a real-life Downton Abbey waiting to happen: for no reason but that they are women, three direct descendants will be skipped over.

"It's totally unfair," drawls the Earl. "But unfortunately, that is what has to happen." It's an injustice that generations of British women have had to put up with. Titles have zig-zagged outrageously across family trees in pursuit of male heirs, or simply died out, because women are, by statute, considered unsuitable inheritors. It is one of the last pieces of gender discrimination enshrined in common law.

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No wonder a wave of anger is rising up against it. Change has been mooted before, but peers have generally preferred to leave the status quo, fearing that drawing attention to themselves could lead to the banning of hereditary peerages altogether. But now a sudden and vigorous campaign for change has swung into action, triggered by the introduction of the Succession to the Crown Act, which has been hurried through Parliament to allow Prince William's first child, due next month, to inherit the throne, whatever the gender.

When the bill was debated in the House of Commons in January, MPs raised the question of what implications it would have for primogeniture among peers. Then last month, more than 200 signatories, drawn from some of Britain's grandest families, called for an equivalent amendment to be made to the laws governing the inheritance of dukedoms, earldoms and marquisates.

It might, at first, seem a matter of footling importance, given how few people it affects – roughly 1,000 families. But an energetic campaign by a handful of titled women to end centuries of discrimination has sent shockf waves through the aristocracy, and has forced the issue into parliament. Two separate bills have been introduced in recent months: Lord Lucas has tabled a private member's bill in the Lords, while Mary Macleod MP, the Conservative member for Brentford, has a Ten Minute Rule bill.

The campaign, spearheaded by Lady Liza Campbell, the Countess of Clancarty, the Hon Sarah Long and the Earl of Balfour, has gained the support of more than 20 MPs and has an active Facebook page, called Equality for Women in the Peerage. Its members call themselves "the hares", after Lord Trefgarne said the Royal succession Bill had "set running the hare of what happens to the hereditary peerage". They even have a logo: a gold hare brooch.

So far, both bills have had their first readings, a formality, and are awaiting a date to be set for their second readings, which will be the first opportunity for a debate. Explaining her reason for championing her bill, Macleod said: "This is about much more than titles and the aristocracy; this is symbolic. It is about the principles of fairness and equality". Now that change is not just a fantasy but a possibility, noble families have been forced to address the elephant in the drawing room. And perhaps not surprisingly – given that money and land is at stake – not everyone is in favour.

Most vocal among them is Edward Lambton, 'Ned' to his friends, or the 7th Earl of Durham to readers of Debrett's. As the only son of the late Lord Lambton, the Tory minister who resigned in 1973 after being photographed in bed with a prostitute, he inherited the entirety of his father's £12m fortune, including Lambton Castle and Biddick Hall in County Durham, and Villa Cetinale in Tuscany, considered by some to be Italy's most beautiful house. As the elder Lambton spent the last 30 years of his life in Italy, under Italian law all his children are entitled to a share of the estate. Since his death in 2006, three of his daughters have been locked in a dispute with their brother, demanding their share. Just as they were preparing to settle for a payment of £1m each, negotiations broke down and Ned served a High Court writ, designed to clarify some outstanding legal issues.

A few days before, Durham took to Facebook to attack the equality campaign. "It is stupid in my view not only to be battling for something that could only possibly appeal to somebody's pride and vanity, but also something that affects about 0.0001 per cent of the population and is therefore something of such little significance as to be not worth fighting for," he wrote.

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"As for property and money, people can leave that to whoever they want, and there is no law whatsoever that says that inheriting a peerage is automatically tied to inheriting property, which is an entirely separate matter. The other thing is that it is normal all over the world for a wife to take her husband's name, and this is what you will, in effect, be trying to change, and this is what people will not like, unless they are trying to ensure that their family name dies out. I am a peer and it's never been of any use or significance to me whatsoever. You may as well fight for the rights of ants to spell their name with a capital A."

Few peers have been quite so outspoken in their criticism of the proposed changes, though some have expressed a quiet concern. Earl Fortescue says changing the law would cause "enormous complications", and he thinks it unlikely to happen. "I can't see Government wasting an awful lot of time worrying about someone like me."

The Hereditary Peerage Association, formed in 1999 to protect peers' interests after all but 92 lost their seats in the Lords, debated the possibility of change in 2008. Most who contributed were in favour of allowing women to inherit, though the complications of amending the law were considered a barrier. "I think this is a case where sleeping dogs are best left to lie," cautioned Lord Coleraine, a London-based solicitor. "I think it would be very unwise to attempt to interfere with the devolution of peerages created in the past," added Lord Saltoun of Abernethy. "Every instinct tells me that once that cat is out of the bag, a whole lot of other cats, which we hadn't realised were there, will follow."

Passionately arguing for change was one Mr Julian Fellowes, now Lord Fellowes of West Stafford. Of anyone, he has done most to bring this issue to the public's attention, making it the central plot device of his Bafta-winning blockbuster Downton Abbey. As its nine million viewers know, the story centres on the plight of the Earl of Grantham who, like the real-life Earl Fortescue, has three daughters and no son. The estate and title is entailed exclusively to a male heir, so it has to pass to a distant cousin, Matthew Crawley, played by Dan Stevens.

The issue is close to Fellowes's heart, as his wife, Emma Kitchener, is the great-great-niece of Field-Marshal Herbert Kitchener, who was created 1st Earl Kitchener at the start of the First World War, and is famous as the face of the 'Your Country Needs You!' posters. The current Earl Kitchener, Emma's uncle, has no children, so the title will die out with him. In an interview with the Radio Times, he said: "If you're asking me if I find it ridiculous that… a perfectly sentient adult woman has no rights of inheritance whatsoever when it comes to a hereditary title, I think it's outrageous, actually."

He added: "Either you've got to get rid of the system or you've got to let women into it. I don't think you can keep it as 'men only'. The point is not whether or not you approve of hereditary titles, but given the fact they do exist, the exclusion of women from them under English law is absolutely bizarre." In October 1998, the Fellowes all changed their names by deed poll to Kitchener-Fellowes, to preserve the name through their son Peregrine, 22, an actor and playwright, like his father.

In the 2008 debate, Fellowes suggested that the exclusion of women from succession could in fact already be illegal. "No one has yet tested it in the European courts but when somebody does, as they will, there is every likelihood that the judges there will rule it contrary to European law."f

Despite that, the Government has sought to downplay the campaign, even while rushing through the Royal Succession Act. A Number 10 spokesman says: "Changes to the law on succession to the Crown can be effected without any change to the legitimate expectations of those in the line of succession. Changes to the rules governing succession to hereditary titles would be far more complicated to implement fairly."

Certainly, some technical questions would need to be addressed. Peerages are created by means of letters patent, an ancient form of legislation that comes straight from the monarch, without the need to consult Parliament. The standard wording used for the creation of most peerages specifies that the titles should pass through the male line, so they would all in effect need to be torn up and rewritten. Fellowes says this is not insurmountable. "When the Duke of Marlborough was needed for another campaign and his only son was dead, a bill was introduced into Parliament granting a new remainder allowing a unique form of female descent to the existing title, without recreating it. The dukedom still dates from its first creation, despite the later, altered, female-friendly remainder."

There is also the question of whether the new rules of succession would work retrospectively. Fellowes says they needn't, as new documents could specify that peers would be confirmed in their title starting from a specific date. And what of the husbands of women who inherit – would they be obliged to take their wives' name?

All this has yet to be decided, and is likely to be the subject of intense debate in both Houses of Parliament. Were such a bill to be passed, it would have a profound effect on the future of Britain's aristocracy, an institution some might argue is pointless and anachronistic anyway. But there is an element of romance amid the folly – titles are a connection to our past. Hundreds have come and gone over the centuries, but they are not being replaced. Only six hereditary peerages have been created since 1965, three of them within the royal family.

The paradox at the heart of the debate is that those who oppose change do not want to meddle with tradition. And yet, allowing women to inherit would ensure titles need never die out. Perhaps the most poignant example is that of the Knight of Glin who died in 2011, leaving no son but three daughters. As the 29th Knight, also known as the Black Knight, Desmond FitzGerald's lineage could be traced back to 1260. His eldest daughter, Catherine, was married briefly to Ned Durham, but is now married to the actor Dominic West, by whom she has three children. She is one of four women who give their views in the coming pages. Had the law changed prior to her father's death, there would still be a Knight of Glin today. As to what Sir John FitzJohn, the 1st Knight of Glin, would have made of a woman becoming the 30th Knight, or how she might have been addressed – we can only imagine.

MARY MACLEOD

Conservative MP for Brentford and Isleworth; has tabled a Ten Minute Rule Bill on the issue

"It doesn't matter that it only affects a few people, I still think that this is something to which we need to say no, this must change. I never approach anything saying anything is too difficult.

Yes, it will be complex, and it won't necessarily be easy, but that's no reason to say we shouldn't change it. So let's have a proper consultation on it, where we understand all the complexities.

But there's no reason why it can't be changed. If we can change it for the monarchy, we can change it for the rest of society. Lord Lucas is doing a private member's bill in the Lords, and then what I'm doing in the Commons is trying to garner as much support as possible, just getting their views.

The vast majority of people I've spoken to about it, Members of Parliament, have said they are absolutely supportive of it.

It may not be their top priority, but a lot of people were surprised – they didn't realise that this still existed, they assumed that with time it would have changed. So I'm glad I've been communicating to people about it, because it's worrying when people don't realise things like this.

Most people who are for general fairness and equality in society are saying yes, it absolutely makes sense to change that.

The role of women has changed now – we fought so hard for women's rights in so many ways, that it seems wrong that girls and women cannot inherit estates.

For those people who may be perhaps more traditional and think that we don't need to change it, we have to say, well if you want to keep history and tradition, the way to do it is to allow women to inherit."

CATHERINE FITZGERALD

Eldest daughter of the late Knight of Glin; married to the actor Dominic West

"My father was the 29th and last Knight of Glin, a title that descends through the male line only, so with three daughters and no other legitimate FitzGerald claimants, it has become extinct.

The Knights have lived on the banks of the Shannon estuary for 700 years, weathering many a reversal of fortune. The survival of the title until recently provided a direct link and insight into a colourful past full of blood-curdling adventure: a microcosm of the nation's own history played out by one family.

In our small region we have recently lost several other ancient titles, like the McGillycuddy of the Reeks or the Earls of Dunraven of Adare [who had no sons and one daughter].

These ancient titles are a symbol of longevity in a shifting world. Some might say there are more urgent matters to deal with, that these titles are irrelevant to modern life. I would argue their loss is a decimation of history. How long will it be before other titles such as The Knight of Kerry and The O'Grady of Kilballyowen go the same way? How meaningless! Just because women cannot inherit? Have we not moved on from this position by now?

People ask me passionately how the title can be allowed to die out. And why I don't get off my butt and do something about it. Why indeed? After all, our FitzGerald family war cry is 'Shanid a Boo' (Shanid being their first stronghold) and means 'Shanid forever' in Gaelic."

LADY LUCINDA LAMBTON

Eldest daughter of the late Lord Lambton; writer and broadcaster

"Gender discrimination – they are two words I would never dream of using but there is no other way to describe it. My brother was left whatever it was, £200m, and we were left not even a pencil.

The buildings and countryside of England are the very lifeblood of my being, and I feel very, very strongly about it. I love the north-east with all my heart, and the people who live there, who I have known since I was a baby. So yes, I do mind not inheriting.

But I haven't ranted about it before because I've accepted that this is the way that the very tiresome English cookie crumbles. But the law is changing now.

In fact Ned, my brother, once said to me, 'I feel very guilty I'm having all this and you're not', and I said, please, speak not of it. But it is a monstrous injustice.

One enlightened man is Hugh Cavendish, who has handed over Holker Hall in Cumbria to his daughter. It shows girls can do it just as well."

LADY TANYA FIELD

Eldest daughter of the Earl of Macclesfield; works for a mental health charity

"My father is the Earl of Macclesfield. If he were to die, the title will go to his younger brother, and after him it would go to his son, my cousin. It's basically sexual discrimination – it's as clear as that.

"It has taken a while for women generally to get equal rights, and while this is one of those things that doesn't directly affect every woman, it just sets a culture, that this is acceptable, that women are second-rate.

"I don't think I know anyone who disagrees – it's archaic. So I'm hopeful that we can change it – you're always surprised what people can do when they really want to get something achieved. So many times you hear people being told, 'You can't do that'. Well actually, if you really put your mind to it, you stand a pretty good chance.

"In my family, the money was carved up by my great-grandfather in a different way, so that it stopped coming down to the eldest son. So it would make no financial difference to me, or, if I'm honest, in my everyday working life.

"But it's just that cultural inequality I would like changed. I think it's wrong for the Government to sit back and accept an inequality like this."