"Their performance is deteriorating and Reform’s is getting better. Inevitably, some groups are showing a Reform lead.”
It comes amid intense speculation over whether Nigel Farage will make a return to frontline politics.
Pollsters have suggested a comeback by Mr Farage would provide a boost to Reform UK.
Mr Anderson became the party's first MP when he defected earlier this month after loisng the Tory whip in an Islamophobia row.
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