Rishi Sunak’s proposals to strengthen the government’s anti-terrorism programme risk “straying into thought crimes” and are potentially damaging to national security, a former senior police chief has said.

The former chancellor announced measures to beef up the Prevent programme on Tuesday night, as part of a bid to boost his flagging campaign to succeed Boris Johnson as the next prime minister.

These would lead to more people being referred to Prevent by widening the definition of “extremism” to include those who “vilify” Britain, with Sunak pledging to focus on “rooting out those who are vocal in their hatred of our country”.

But former counter-terrorism chief Sir Peter Fahy, who was also chief constable of Greater Manchester police, questioned the precise meaning of “vilification”.

He said: “The widening of Prevent could damage its credibility and reputation. It makes it more about people’s thoughts and opinions.

“It is straying into thought crimes and political opinions.”

He added: “Political opposition is not where police should be, it is those who pose a serious threat and risk of violence, not those opposed to political systems.”

Sunak said on Tuesday night that “Britain is a beacon of freedom, tolerance and diversity,” warning against ever “letting those who seek to undermine and destroy our way of life to succeed”.

He said he would refocus the Prevent programme to tackle Islamist extremism, what he called the UK’s “most significant terror threat”.

He also promised to “weed out and cut off organisations that are promoting extremism in the UK”, and added: “There is no more important duty for a prime minister than keeping our country and our people safe.”

Extremism is defined in the 2011 Prevent strategy as “vocal or active opposition to fundamental British values, including democracy, the rule of law, individual liberty and mutual respect and tolerance of different faiths and beliefs”, as well as calls for the death of armed forces personnel.

Sunak’s team vowed to broaden that out to include “vilification of the UK” to ensure “those with an extreme hatred of our country that leads them to pose a risk to national security can be identified and diverted away from a destructive path”.

They stressed it was not a legally binding definition and simply criticising the government or any of its policies would not count as vilification but instead “help guide the public sector in its work to keep the UK safe”.

Prevent previously ran into controversy over whether it should cover those criticising UK foreign policy, and British conduct in the 2003 Iraq war. In recent years the police side of Prevent has tried to focus much more on those at risk of falling into terrorist violence.

Those in the public sector are under a duty to report concerns to Prevent, which has previously caused consternation. Their information is the lifeblood of the counter radicalisation scheme, which has been dogged by claims it has strayed in suppressing freedom of thought and speech.

Fahy said: “The danger is the perception it creates that teachers and health workers are involved in state surveillance.”

“What does vilification mean? Vilification would have to be carefully defined.”

Former counter-terrorism chief Neil Basu said that Prevent was the most important strand of the UK’s battle about violent extremism. A review of the programme ordered by the government has been delivered to officials but there is no public timetable for its publication.

Sunak’s pledges to improve security include blocking terrorists from trying to “abuse our human rights framework” by classifying them differently from the general prison population via a new bill of rights; auditing publicly-funded third-party organisations to ensure no extremist groups gets taxpayer cash; and funnelling off those referred to Prevent who are more in need of mental health support.

“Whether redoubling our efforts to tackle Islamist extremism or rooting out those who are vocal in their hatred of our country, I will do whatever it takes to fulfil that duty,” he said.