Boris Johnson has branded climate change activists who have blocked motorways and caused disruption in recent weeks “irresponsible crusties”.
Speaking to LBC, the prime minister said the those involved are “doing considerable damage to the economy” and are “not” legitimate protesters.
Mr Johnson’s father, Stanley Johnson – who is also an environmental campaigner and international ambassador for the Conservative Environment Network – told Sky News that the actions of Insulate Britain protestors are not helping their cause.
It comes as Home Secretary Priti Patel is expected to warn protesters who block the motorway could face unlimited fines and up to six months in jail during her conference speech on Tuesday.
Police will also be given powers to stop and search activists for “lock-on” equipment used to prevent them from being moved, it is believed.
The move follows days of protests by Insulate Britain, which has staged sit-down demonstrations on a series of key roads around London – including on the M25, M1 and M4.
On 2 October, the government obtained a fresh injunction banning the group from obstructing traffic and access to motorways and major A roads around London.
The new powers will be introduced by Ms Patel and added as an amendment in the Police, Crime, Courts and Sentencing Bill, currently going through parliament.
The PM’s father told Sky News that “Extinction Rebellion and Insulate Britain are making a mistake if they think antagonising ordinary people is going to help the cause”.
He added: “It is not a good idea, it needs to stop – whether it needs to stop by legal means – Kit Malthouse or whatever – I do not know, but I am simply saying as a matter of practical politics those who lead Extinction Rebellion and Insulate Britain need to realise that this is not actually going to advance the cause they are seeking to support.”
Ahead of her conference speech, Ms Patel said: “The right to protest is a fundamental principle of our democracy but we will not tolerate guerrilla tactics that obstruct people going about their day-to-day business.
“Whilst the Labour Party stand on the side of these so-called ‘activists’, the Conservative Party will always back the law-abiding, hard-working majority of people in this country.”
The PM added that his government “will give the police the powers they need to stop their reckless and selfish behaviour”.
Speaking earlier on Sky News, Policing Minister Kit Malthouse said Insulate Britain protestors have “crossed the line” of peaceful protest after video footage was released showing a paramedic removing an individual from the middle of the road.
Describing the footage as “deeply distressing”, the policing minister said something “does” need to be done about the situation.
“While we obviously all value the right to protest, there is a difference between causing disruption and causing damage,” he said.
“We believe that these protesters and some of the others that we have seen in the last couple of years have crossed the line between exercising their right but also their responsibility towards the rest of us and something needs to be done.”
Mr Malthouse added that the government will be “announcing a raft of new measures” to curb protests such as this later on Tuesday.
Ms Patel is due to make her conference speech in the main hall at 11.50am.