Plans to introduce laws in Britain to combat money laundering could prevent the media and researchers from accessing vital information on the tide of dirty cash flowing around the world, campaigners and journalists have warned.

An open letter to the foreign secretary, Jeremy Hunt, has about 80 signatories including journalists from the Guardian, which reported the story. They say the government’s approach to a new register of beneficiaries of obscure financial structures risks undermining previous efforts to combat illicit financial flows.

The register is being updated as a result of the passage of the fifth anti-money laundering directive (5MLD), passed by the EU after the Panama Papers revelations, which were reported by an international collaboration of news organisations including the Guardian.

Proposals published by the Treasury this year say journalists and researchers will have to present evidence of wrongdoing before accessing the register.

The letter’s signatories have argued that by submitting evidence to HMRC to determine whether access to the register should be granted, it is possible that HMRC could trigger an investigation, which can often take many years, during which time it is unlikely that any journalism or research could be published.

The letter, sent by the anti-corruption group Global Witness as the first global conference for media freedom opens in London, reads: “The success of this summit offers the opportunity for the UK government to set the gold standard for an environment that enables journalists to hold the powerful to account.

“Sadly, the government’s current approach to implementing a key piece of legislation to combat money laundering – the fifth anti-money laundering directive – does not meet this standard. It appears designed to frustrate journalists and researchers from accessing and ultimately reporting on vital information about the beneficiaries of the opaque system of financial trusts that have tax liabilities in the UK.”

The letter adds: “The UK government’s proposal for transposing this directive into national law restricts this legitimate interest to see and report on this information by requiring journalists and researchers to present evidence of wrongdoing before accessing the register, completely undermining the objectives of 5MLD.”

The coalition of NGOs and journalists said it was also concerned by the negative connotations that the UK government attached to journalists and researchers using the transparency created by the legislation to make “speculative enquiries”.

The Panama Papers comprised 11.5m files leaked from the Panama-based law firm Mossack Fonseca. The documents were leaked to the German daily Süddeutsche Zeitung, which shared them with the Washington-based International Consortium of Investigative Journalists, the Guardian, the BBC and other media organisations.

Among revelations reported in the Guardian was a network of secret offshore deals and vast loans worth $2bn (£1.4bn) that lay a trail to Russia’s president, Vladimir Putin.