Theresa May’s vow to rip up human rights laws if necessary to tackle terror suspects would involve declaring a state of emergency, European court of human rights documents confirm.

Damian Green has confirmed that such changes to human rights laws would involve “a derogation” from the European convention on human rights. The cabinet minister said France had taken similar action in the aftermath of the Paris attacks.

Labour’s Keir Starmer, a former director of public prosecutions, has disputed that human rights laws prevented tackling terrorists or terror suspects.

“There is nothing in the Human Rights Act that gets in the way of effectively tackling terrorism. I can say that with this authority. I was director of public prosecutions for five years. I’ve worked very closely with the security and intelligence services and we’ve prosecuted very, very serious criminals and the Human Rights Act did not get in the way of what we were doing,” he told the BBC.

“This is a diversion … We’ve had three terrible attacks in three months. The problem is people just coming on to the radar, then the question of how they are risk assessed, and what resource we’re putting in. And the prime minister, because she was facing searing questions about that yesterday, about resources, she has now brought up the Human Rights Act as if that stands in the way of the current problems,” he said.

A European court of human rights factsheet stresses that “the right to derogate can be invoked only in time of war or other public emergency threatening the life of the nation”.

May has denied that her pledge to rip up human rights laws to bring in a tougher regime of restrictions on terror suspects and deportations contradicts her manifesto pledge not to withdraw from the European convention on human rights in the next five years or repeal the Human Rights Act before Brexit.

Ministers claim that “derogation” – involving the partial and temporary withdrawal from the European human rights convention – is consistent with that manifesto pledge.

But the European court of human rights document states that this can only be done by declaring a state of emergency and that derogations can only be made “to the extent strictly required by the exigencies of the situation”.

Their factsheet also points out that no derogation is allowed from certain convention rights, including the rule of “no punishment without law” as well as bans on torture and inhuman or degrading treatment or punishment. The latter right may rule out deportation deals involving terror suspects with countries that May wishes to strike to speed up their removal from Britain.

The only other time that Britain has “derogated” from the European convention on human rights in recent times was in the aftermath of 9/11, when the regime of indefinite detention without trial of international terror suspects in Belmarsh high-security prison in London was introduced. Other derogations took place in relation to the use of emergency powers in Northern Ireland.

It was eventually ruled unlawful in 2004 by law lords on the grounds that three years after it was introduced there was no longer a state of emergency threatening the life of the nation.

May said on Tuesday night: “If human rights get in the way of doing these things, we will change those laws to make sure we can do them.”

This flatly contradicts the commitment in her manifesto not to rip up or amend the Human Rights Act before Brexit, which is at least two years away, or to withdraw from the European convention on human rights for at least the next five years. Derogation, which would be needed, amounts to a temporary and partial withdrawal from the convention.

The manifesto says: “We will not repeal or replace the Human Rights Act while the process of Brexit is under way but we will consider our human rights legal framework when the process of leaving the EU concludes. We will remain signatories to the European convention on human rights for the duration of the next parliament.”