Sarah Cooper-Lesadd has quit Reform UK and joined Plaid Cymru just four months after being elected to the Senedd, accusing her former party of being “not serious about governing Wales”.
The Member of the Senedd for Pen-y-bont Bro Morgannwg announced her defection on 15 September after Plaid Cymru's Senedd group and National Executive approved her admission to the party.
The move takes Plaid Cymru to 44 members in the 96-seat Senedd and reduces Reform UK, the largest opposition group, from 34 to 33.
Cooper-Lesadd had served as Reform UK's Welsh shadow minister for children, young people and skills following her election in May.
Cooper-Lesadd says Reform is 'not serious about governing Wales'
Explaining her decision, Cooper-Lesadd said she entered politics to represent care-experienced people and others she believed were being overlooked.
She accused Reform of having “no interest in the work I came here to do” and said the party was “far more interested in division, opposition and headlines than in finding solutions”.
Those comments represent Cooper-Lesadd's assessment of her former party.
She also claimed there was an appetite within the Reform group for the Welsh Government to fail, regardless of what that would mean for people in Wales.
Disagreements over budget and asylum policy
Cooper-Lesadd identified several areas in which she had disagreed with Reform's approach in the Senedd.
She said she wanted to support the Welsh Government's supplementary budget and had argued against Reform tabling a debate on Wales's Nation of Sanctuary policy.
She said she could not support what she described as Reform's attacks on people seeking sanctuary and other vulnerable groups and objected to minorities being used as political targets.
These criticisms are Cooper-Lesadd's political claims about Reform and have been attributed to her rather than presented as established fact.
Claims Reform sees Senedd as route to Downing Street
Cooper-Lesadd also criticised Reform UK's national leadership, claiming the party centrally had “no interest in Wales”.
She described Reform's Senedd group as a “staging post” in an effort to get Nigel Farage into Downing Street and said she believed that had become particularly apparent at the party's conference.
Addressing people who voted for Reform in May, Cooper-Lesadd said she had not stopped listening to them and argued their concerns should be answered through delivery rather than division.
Reform says it planned to expel her
Reform UK disputed the circumstances surrounding Cooper-Lesadd's departure.
A Reform Wales spokesperson said the party had been planning to expel her because of her support for Welsh independence.
That represents Reform's account of events. Cooper-Lesadd announced publicly that she had left the party and joined Plaid Cymru.
Plaid approves defection
Plaid Cymru Senedd group chair Marc Jones said both the parliamentary group and the party's National Executive had agreed to admit Cooper-Lesadd following an internal process.
Jones acknowledged that accepting a politician elected for Reform was not a decision taken lightly, particularly because Plaid members had campaigned against Reform candidates only months earlier.
He said Cooper-Lesadd had concluded she could no longer make the difference she wanted from within Reform and welcomed her to the Plaid Cymru group.
Cooper-Lesadd said she understood the concerns some Plaid members might have about her move and promised to work to earn their trust.
Defection follows May election
Cooper-Lesadd was elected for Pen-y-bont Bro Morgannwg in May 2026 under the Reform UK banner.
She was subsequently appointed Reform UK's Welsh shadow minister for children, young people and skills.
Her defection means Plaid Cymru now has 44 MSs, five short of an overall majority in the 96-member Senedd, while Reform retains 33 and remains the largest opposition group.
Another turbulent day for Reform in Wales
The defection was announced on the same day Dan Thomas stood down as leader of Reform UK in Wales after being arrested at the weekend on suspicion of assault and engaging in controlling or coercive behaviour.
Thomas denies wrongdoing and has not been charged. Gwent Police said he had been released on bail pending further enquiries.
The two developments leave Reform needing to choose a new Welsh leader while simultaneously adjusting to the loss of one of the 34 MSs elected under its banner in May.