This week, the World Association of Newspapers and News Publishers has dispatched a delegation of editors and publishers from as far apart as Pakistan, Canada, Denmark and Uruguay. Its mission is not to some far-off nation or dictatorship, but to the UK. The mission was originally motivated by concerns over the royal charter and the planned new system for press regulation. The delegation's agenda quickly expanded to include an examination of the hostile response to the Guardian's reporting on the Snowden leaks, which in many ways is far more worrying.

There has been considerable controversy over press regulation in England and Wales. Although effective self-regulation should always be the preferred method of press regulation, other models are compatible with international law provided they include adequate safeguards for press freedom. For example, Denmark, Ireland and Finland all have regulatory mechanisms premised on legislation that provide effective support for a robust and independent press.

The current system has failed to ensure the accountability and transparency of the press. The phone-hacking scandal made this quite clear and underscored that change was needed. Now that we have change, which on the face of it provides effective safeguards against political interference, parties should focus their energy on making the system meaningful and functional. The government must be clear in its message that the charter does not usher in political control, but safeguards against it.

In addition, it must be a committed vocal advocate for press freedom. Sadly, such commitment has been lacking in recent months. The reaction to certain stories in the Guardian are a case in point.

The organisation's reporting of the Snowden leaks in its paper and online revealed mass surveillance of the digital communications of millions of people by the security services, including GCHQ. Through its serious-minded, public-interest journalism, the Guardian has initiated an important debate about the appropriate balance between national security and individual liberties.

Rather than addressing the issue at the heart of the scandal (secret state spying on the public) the government, no doubt embarrassed, has sought to shoot the messenger. The hostility shown towards the Guardian has been almost palpable. The paper has faced censorship, harassment and the threat of legal persecution.

Officials visiting the Guardian offices asked the organisation to refrain from publishing further information about the leaks and for computer hard drives to be destroyed. Such requests are an unacceptable interference with press freedom.

David Miranda, who was working with his partner, the then-Guardian journalist Glenn Greenwald, on the leaks, was detained at Heathrow airport and his laptop computer seized under schedule 7 of the 2000 Terrorism Act. Regardless of whether the detention was lawful, under broad powers allowing detention without probable cause, the move seems nothing short of harassment designed to prevent or discourage reporting, rather than a genuine concern relating to terrorism.

Some politicians, including Liam Fox and Julian Smith, seem determined that the Guardian should be prosecuted under the Terrorism Act, which makes it an offence to communicate information that might aid terrorism (a problematically vague and wide category) and which does not reasonably allow a public interest defence (which the Guardian should certainly have).

Politicians should be addressing the questions the public want to answered, rather than attacking the media organisation raising those questions. What is the scope of this secret mass surveillance? Is political authorisation of surveillance rather than independent judicial approval acceptable? Is the current oversight mechanism adequate? The answer to the latter two issues, I contend, is no.

It is indeed "unprecedented" that an international press freedom mission should be sent to the UK – their own words, not mine. Such delegations are typically found addressing concerns in some of the most challenging environments for journalism around the globe, where the anti-censorship organisation Article 19 usually focuses its efforts.

The mere appearance of such a group on British soil should sound alarm bells. Whether it does remains to be seen. Along with other free-expression groups, I will meet with the delegation this week. My message will be clear: the long tradition of the UK's commitment to fundamental rights and freedoms appears to be in danger. Every effort must be made to preserve it.