Nigel Farage has suggested UKIP's Neil Hamilton may be too old to be a frontline politician.
67-year-old Hamilton re-ignited his political career last week, winning a seat on the Welsh Assembly.
He then went on to successfully challenge Welsh party leader Nathan Gill for leadership of its seven-strong group of AMs.
But his return was met with hostility by the national party leader Nigel Farage, who said the treatment of Mr Gill had been 'unjust' and an act of 'deep ingratitude'.
In a fresh attack, Mr Farage questioned whether Mr Hamilton was young enough to take on the role.
Speaking to LBC radio he said Hamilton was 'getting on a bit'.
Earlier this week the secretary of UKIP Wales resigned, saying he was 'appalled' by the appointment of the former Conservative AM.