Strict rules are the solution to political corruption. But sometimes, they can also be the problem. Powerful elites have a habit of exploiting legal loopholes to hide dark money.

This was something I learnt the hard way in 2015, when I travelled to the seaside town of Lancing to try to dig up the financial secrets of British politicians.

It turns out that Lancing is home to a remarkable database that only a small handful of people have seen. It contains the names of every individual who holds shares in Britain’s biggest companies, showing exactly how much money they have invested in the likes of Tesco, Rolls-Royce and BAE Systems.

For most companies, this information is submitted to Companies House and published online, where it is freely available for anyone to see. But when a business is listed on the stock exchange, a different set of rules kick in – instead of publishing online, the list is recorded in a secretive database.

The only way of viewing it is to make a formal written request under the Companies Act 2006. If your request is accepted, you can either pay £95 for a copy to be emailed to you, or you can visit a small office in this West Sussex town, where the database is pre-loaded on an ancient desktop computer.

I didn’t just want to look at one company’s list of shareholders: I wanted to look at every FTSE 100 company. That would have meant a bill of £9,500. And so I got on the train to Lancing.

There is a clear public interest in exposing the secret shareholdings held by MPs. When politicians vote against windfall taxes for BP, we should know if they have a financial stake in the company. If a minister brokers an arms deal with Saudi Arabia, we should know if they stand to make money from it.

And even if these conflicts of interests continue, transparency is the minimum requirement.

Yesterday, the Guardian published a series of investigations that did just that, and – for the first time – the undisclosed shareholdings of MPs were made public. They included investments held by the attorney general Victoria Prentis, former prime minister Theresa May, and a host of other senior politicians.

Yet none of these politicians has acted unlawfully. Instead, their shareholdings have been hidden by a series of outdated laws and rules that make this kind of accountability almost impossible.

Written into the Companies Act 2006 is a clause that says anyone requesting to view the shareholder register must first declare whether they intend to share the information with anyone. If they do, they must provide the names and addresses of those people (as well as their own).

In effect, this means any journalist who wants access to the database needs to guess the names and addresses of anyone who might potentially read their article. Obviously, this is impossible.

A further clause says it is a criminal offence to make a “misleading, false or deceptive” request – say, one that did not disclose an intention to use the material for publication. A person found guilty of doing this could be jailed for two years. All of which means that investigative journalists are essentially blocked from this crucial database, both financially and legally.

In its report, the Guardian says it worked with lawyers for six months to secure access to the shareholder database – suggesting it may have found a legal argument to navigate this. One approach they may have considered is to cite the discussions that took place in Parliament when the Companies Act was first introduced.

The Labour minister representing the government as the act made its way through the Commons, Margaret Hodge, said at the time: “I think that a judge, or whoever will consider those issues in court, will be able to distinguish between legitimate investigative journalism, which we wish to protect, and illegitimate haranguing of [shareholders] by extremist thugs.”

But there is nothing written into the legislation that specifically marks out investigative journalism as having a special exemption. So although it is possible that Hodge’s comments from 2006 are enough to persuade companies to disclose their shareholder database, this is far from guaranteed. Other journalists wishing to investigate this further will still need to navigate an extremely risky legal minefield.

Nor is this the only obstacle to transparency about MPs’ financial interests when it comes to shares in big companies.

In the House of Commons, MPs only have to declare shareholdings if they are “valued at more than £70,000”, or if they own more than 15% of a company. That means most of the MPs named by the Guardian do not have to declare their shareholdings in Parliament. This means that – hypothetically – Rishi Sunak could hold a £69,999.99 investment in the energy giant Centrica, and would be perfectly entitled to keep it a secret.

Are we really meant to believe that this would not be a huge conflict of interest? And should journalists who expose this really face the threat of prison?

When Sunak became prime minister, he promised “integrity, professionalism and accountability at every level”. But all we have seen is secrecy, lobbying and dark money. Of course, some of these issues (like the Companies Act) long predate his time in office – but Sunak is waging his own war against public scrutiny.

Sunak’s government took openDemocracy to court in a failed bid to avoid handing over its secret internal Covid review. And the government’s National Security Bill threatens to crush press freedom by making it an offence for anyone to provide reporters with any information that may “materially assist a foreign intelligence service”.

If the government was serious about integrity and accountability, it would stop creating obstacles for investigative journalism. And it should rewrite the existing rules that make it almost impossible to spot MPs’ conflicts of interests.