Gwent Police said: “On Sunday 13 September a 45-year-old man, from the Aberbargoed area, was arrested on suspicion of assault and engaging in controlling/coercive behaviour.
“He has been bailed pending further enquiries.”
Ken Skates, leader of Welsh Labour, told the Press Association: “This is quite incredible news and many questions will need to be answered.
“Dan Thomas was a personal pick by Nigel Farage and this will surely demonstrate the appalling judgement of the Reform UK leader.”
Mims Davies, the Conservative Shadow Secretary of State for Wales said: “Reform are hitting the news for all the wrong reasons – massive crypto donations, on top of the jailing of Nathan Gill for pro-Russian bribery.
“All this raises serious and deep questions about Reform’s judgement and leadership at every level.”
Welsh Liberal Democrat Leader, Jane Dodds, said: “The wheels are rapidly coming off Reform in Wales and the entire country.”
Thomas grew up in Blackwood in the south Wales valleys.
Both his grandfather and great grandfather were miners, he said, and he had “fond memories” of growing up in Wales – but left when he became an adult because “good opportunities” there were “few and far between”.
He worked in financial services in London and became a councillor, and later Conservative leader, at Barnet council in the north of the capital.
During this time Thomas stood for the Conservatives in the Islwyn constituency at the 2010 and 2017 general elections, coming second to Labour’s Chris Evans on both occasions.
Thomas then led Barnet council from 2019 to 2022, eventually becoming opposition leader after Labour won the local election there until May 2024.
In June 2025 Thomas defected from the Conservatives to Reform, and at the end of the year resigned from the council.
Local media quoted him saying he moved away from Barnet so that he and his wife could raise their young sons in the countryside and live closer to their families in south Wales.