A retirement age of 80 will be imposed on House of Lords members if Labour wins the general election, the party has said.
Sir Keir Starmer has also vowed to phase out the remaining hereditary peers - Lords who inherit a seat in Parliament from their parents - in Labour's manifesto, published on Thursday.
The Labour leader had initially vowed to abolish the House of Lords and replace it with an elected second chamber.
But that has been watered down to a long-term ambition. Labour's manifesto now calls for a consultation on replacing the House of Lords with a "more representative" body.
In Labour's plan for government, the immediate priority would be shrinking the size of the Lords and making it easier for "disgraced peers" to be removed.
Participation requirements, to "ensure all peers meet the high standards the public expect of them", would also be introduced.
The size and role of the House of Lords have come under increased scrutiny in recent years, amid warnings that membership has grown excessive with 784 sitting members.
Concerns have also been raised about some of the peers nominated by former prime ministers including Boris Johnson, notably Lord Lebedev.
The media mogul and son of an ex-KGB agent was given a life peerage in 2020, but has spoken just once on the floor of the House.
What is the House of Lords?
The House of Lords is part of the UK Parliament. Also known as the Upper Chamber, it is independent from the House of Commons - where 650 Members of Parliament (MPs) debate and vote on new laws
Members of the Lords are called "peers". Like MPs, they scrutinise the work of government and recommend changes to proposed legislation
Peers can block bills from becoming law for more than a year, potentially gumming up the machinery of Parliament, and most new ones have been approved by the House of Lords
Unlike MPs, peers are not elected. The majority are appointed by the monarch on the advice of the prime minister
While many peers have worked in politics - including some former MPs - others are experts in areas such as science or the arts
Most peers stay in the Lords for life - although some choose to retire
In December 2022, Sir Keir unveiled a report by former Prime Minister Gordon Brown, which contained plans to abolish the House of Lords alongside other constitutional reforms.
The report was billed as a potential blueprint for the first term of a Labour government.
Since then, there have been several attempts to water down the plans, with Labour now promising instead to "immediately modernise" the upper chamber.
In its manifesto, Labour said it "recognises the good work of many peers who scrutinise the government and improve the quality of legislation passed in Parliament, reform is long over-due and essential.
"Because appointments are for life, the second chamber of Parliament has become too big," the document added.
The OceanNewsUK understands the cap would apply at the end of the parliament when peers turn 80, allowing some to stay until the age of 84.
There is no upper age limit for serving MPs.
A mandatory retirement age of 80 would hit Labour's grouping hardest in the Lords, as their peers are, on average, older than Tory or independent peers. There are also fewer Labour peers than Conservative ones.
This could mean a Labour government would need to appoint dozens of new peers in order to be confident of passing new laws through the upper chamber.
Several influential peers would be removed from the chamber under Labour plans.
They include Labour's Lord Dubs, who served as an MP for Battersea after arriving in the UK as a six-year-old fleeing the Nazis, Conservative former Deputy Prime Minister Lord Heseltine and former Lib Dem leader Lord Campbell.
The last Labour government got rid of most hereditary peers in 1999 - but it allowed 92 to remain in a compromise deal.
Their numbers have been maintained through by-elections to replace retiring or deceased members, selecting from 800 peers who inherited their titles.
Several attempts have been made to scrap this system over the years, but it remains in place.
Labour has now promised to introduce "legislation to remove the right of hereditary peers to sit and vote in the House of Lords".
