A Guardian investigation into the House of Lords raises questions over the accountability of parliament’s second chamber, with revelations about how a string of peers are benefiting from commercial interests.

One in 10 members have been hired to give political or policy advice, according to their own declarations, and others do paid work for companies that could conflict with their role as legislators. The findings expose weaknesses in the Lords code of conduct and raise questions about whether the rules on lobbying and paid employment should be tightened in line with restrictions signed up to by MPs.

The investigation sheds new light on the extent to which money flows into politics from those who hold peerages or go on to secure them, with more than £100m given to the three main parties over the last two decades, much of it by a small group of influential super-donors.

Many members of the Lords make a valuable contribution to its main purpose of refining and scrutinising legislation. But their numbers have ballooned to 835 after a succession of prime ministers packed the house with donors and party loyalists. Labour has promised some changes, but there are calls for more ambitious reforms to an institution Keir Starmer has previously described as “indefensible”.

Darren Hughes, the chief executive of the Electoral Reform Society, said: “The Lords should not be a political gated community filled with party donors, as well as friends and supporters of various prime ministers. These revelations again underscore the urgent need for Lords reform so there is far greater transparency and accountability to guard against conflicts of interest, which risk further corroding the public’s already rock-bottom trust in politics.”

Over the coming weeks, the Guardian will publish the Lords debate, a months-long investigation that has involved undercover reporting, and extensive analysis of parliamentary records, political donations and official documents.

It will reveal details of how:

• Nearly 100 members of the Lords are paid to give political or policy advice by commercial firms.

• A Labour peer offered access to ministers during discussions to sponsor an event in parliament.

• A former minister has earned millions of pounds since entering the Lords by working for 30 companies.

• Multiple peers are being paid by foreign governments including repressive regimes.

• More than £1 in every £14 donated to political parties since 2001 came from those who have sat as peers in the last parliament.

Starmer appears to be making slow progress on what many Labour supporters saw as a once-in-a-generation chance to overhaul the UK’s unelected second chamber.

A quarter of a century after Tony Blair tried to reform the Lords by banishing most hereditary peers in 1999, there had been hopes Starmer would use Labour’s return to government to cut the size of the house or replace it with a more representative chamber of the nations and regions, as proposed in a 2022 review by the former prime minister Gordon Brown.

The plan was supposed to address concerns over a lack of equal representation in the Lords, where 69% of members are men. The Guardian’s analysis shows the median age of peers in the current parliament is 71 – four years older than the state pension age – and 51% were privately educated, compared with 6.5% of the population of England as a whole.

At the time, Starmer endorsed Brown’s proposals, promising to transfer power from Westminster to the British people. “I think the House of Lords is indefensible,” the then leader of the opposition said. “So we want to abolish the House of Lords and replace it with an elected chamber that has a really strong mission.”

Ministers are in the process of removing the remaining 92 hereditary peers, amid opposition from many Conservative lords.

However, further changes promised in the Labour manifesto – including an age limit of 80, reforming the appointments process, setting minimum levels of attendance, and a consultation on replacing the chamber – are yet to take shape and there are fears they will be kicked into the long grass.

As revealed by the Guardian last month, the government is looking at striking a deal across the Lords on cutting the size of the chamber without necessarily bringing in an age limit of 80.

However, none of the proposed changes would tighten the rules on lobbying and paid employment.

Peers receive expenses rather than a salary like MPs, and more often hold outside roles, without generally having to declare how much they are paid.

After a 2021 lobbying scandal exposed by the Guardian, MPs are now banned from working as parliamentary strategists or consultants and cannot advise on “public policy” and how parliament works in general. This was not extended to the Lords.

Peers are not permitted to undertake “paid parliamentary advice or services”, meaning they cannot conduct, or advise on, lobbying or try to influence ministers, officials or parliamentarians on behalf of a company that pays them.

However, a number of lords describe their jobs as advisers or consultants for companies, providing political or policy advice, which is permitted under the rules.

One peer, Charlotte Vere, a former Tory transport minister, decided against taking up a job as partner in a lobbying company, Stonehaven, after a public outcry this month. Stonehaven subsequently let go two other peers working for it as advisers.

Gabe Winn, who runs the public affairs company Blakeney and has been speaking out against peers working for lobbying outfits, called for a “complete blanket ban” on peers being paid by lobbyists or as inhouse political advisers, regardless of how their role is described.

“For me, the only answer is a complete blanket ban. Anyone who is a current policymaker in either house or in the civil service should not be able to take any money whatsoever from any agency or consultancy that lobbies on behalf of clients, or inhouse at any company where they’re asked to give advice on politics or policy,” Winn said. “The choice should be simple: make policies on behalf of the public or be paid by companies to influence them – never both.”

The government is still on paper committed to overhauling the rules on appointments to reduce the number of peers selected through political patronage. Each year, party leaders nominate lists of candidates to be approved by the monarch. Many are chosen on the basis of previous service as MPs or in local government, or after making significant financial donations.

Starmer appointed a string of allies and former advisers to the House of Lords earlier this year, including his former chief of staff Sue Gray, the longtime Tony Blair staffer Anji Hunter, and the former TUC head Brendan Barber, in an effort to increase the number of Labour representatives and help get legislation through.

With Reform UK riding high in the polls, there has been speculation that Nigel Farage’s party could seek to get some peers, especially if it wins further seats at the next election. The Green party, which has one fewer MP, has two peers on the red benches.

Reform sources told the Mail on Sunday this month that any deal or pact with the Conservatives at the next election could include an attempt to follow Donald Trump’s example of appointing high-profile figures – similar to Elon Musk in the US – to run government departments from the Lords.