Protected zones coming into force outside abortion clinics represent a balance between religious freedom and a woman’s right to privacy, an MP has said, as an opposition group branded them “chilling and Orwellian”.

Safe access zones will come into effect from Thursday, covering a 150-metre radius where it is expected any act that obstructs or harasses clinic users or staff will be deemed an offence.

The so-called buffer zones in England and Wales were part of legislation passed 18 months ago but there was a delay in implementation while arguments persisted around whether silent prayer should be included.

A group opposing the zones said the inclusion of silent prayer “constitutes a gross intrusion in the right of freedom of religion, free speech” and suggested the measure must be “properly tested in the courts”.

But Labour MP Stella Creasy branded the delay in the zones coming into force “a massive democratic outrage”, because a specific amendment to allow silent prayer in the zones was voted down in Parliament last year when the Public Order Bill was debated.

She told the PA news agency the enactment of the zones this week is “long overdue” and rejected claims from anti-abortion campaigners that the inclusion of silent prayer encroached on their right to religious freedom.

She said: “They (anti-abortion campaigners) have had a democratic moment. Parliament has debated, discussed, and ultimately voted on the very principles that they claim are being denied.

“There could not be a more democratic embodiment of what has happened, which is people have disagreed with them that they have a right to pray wherever they want, and said, ‘well actually, women have a right to privacy, and that privacy means that if you’re going to pray for women having abortions, you need to do it 150 metres away’.

“Nobody’s banning silent prayer, they’re just saying it’s not appropriate here.”

The Society for the Protection of Unborn Children (Spuc) said a ban on silent prayer in the zones is a “chilling and Orwellian measure” to “shut down legitimate peaceful vigils and control religious activity including silent prayer”.

Michael Robinson, Spuc’s executive director, indicated the group could seek legal action as he accused the state of “trying to police silent prayer and thought”.

He said: “We are therefore actively consulting legal experts and believe that the inclusion of silent prayer constitutes a gross intrusion in the right of freedom of religion, free speech and accordingly needs to be properly tested in the courts.”

The Crown Prosecution Service (CPS) is expected to publish guidance for prosecutors while the College of Policing will publish what it described as a brief for officers on section 9 of the Public Order Act which deals with safe access zones.

The Home Office has previously said these will “ensure there is clarity and consistency with the enforcement of the new offence”.

On the wording, Ms Creasy said: “I am very mindful we all need to see the detail. But we do need to be explicitly clear.

“The question of silent prayer was considered in a democratic fashion, and the way to balance the rights of people to pray with the people who want to access an abortion has been found through safe access zones. So I’m expecting the legislation to be very, very clear that the will of Parliament is to be upheld.”

Last month, when announcing the date for the zones to come into force, safeguarding minister Jess Phillips said the Government “will not sit back and tolerate harassment, abuse and intimidation as people exercise their legal right to healthcare, which is why we have fast-tracked this measure to get it up and running without further delay”.