Political commentator Suzy Stride has warned that Reform UK is "concerning because they are gaining ground".
It comes after Reform UK has surpassed the Conservative Party in membership numbers, according to the party's own tracker.
Speaking to GB News, Stride emphasised that Britain has historically not been "a country of extremes".
She said: "Ultimately, Reform UK are concerning because they are gaining ground.
"However, ultimately no one knows because in some ways we are in unprecedented times where we've seen things happen that we haven't seen happen before.
"I think what we can definitely say is that Nigel Farage is someone to watch and to take seriously.
"And, as I always say, the big parties need to be humble and need to listen as to why their people are not voting for them and about voting for Reform."
The insurgent party reached 131,712 members by 11.30am on Thursday, exceeding the last known Tory membership figure of 131,680.
Reform UK leader Nigel FarageGETTY
Reform UK chairman Zia Yusuf called it a historic moment, declaring that "the century-long stranglehold the two old parties have had on Britain is finally broken".
Reform UK's rapid growth has been particularly driven by its focus on attracting younger members.
The party recently reduced its membership fee to £10 for those aged 25 and under.
This strategy appears to be working, with Nigel Farage announcing that 1,000 young people joined the party in less than 48 hours.
Suzy said that the Conservative Party needs to be "humbled"
GB News
Responding to the milestone moment, Farage said: "This is a big, historic moment.
"The youngest political party in British politics has just overtaken the oldest political party in the world.
"Reform UK are now the real opposition."