Job changed after I asked for reduction in hours: ask the expert
It is not clear whether you have over a year's service. If so, then you have the right to resign and claim constructive unfair dismissal in a situation where your employer has fundamentally breached your contract of employment. In your case, it seems that you would have the right to do this (depending on the consultation procedure followed by your employer and the reasons they have given for your role being full time only).
At times, a genuine redundancy situation can arise where there is a need for full-time and not part-time employees, however this is quite difficult for an employer to show. They would need very good reasons for requiring a full time employee. This is going to be difficult to show when you have been doing the job on a part-time basis already.
If you refuse to accept the new job and you are dismissed as a result then I believe you would have an unfair dismissal (again if the procedure followed was not properly done and they could not justify making the role full time).
Tracey Guest is head of employment and a partner at Slater Heelis in Manchester. She specialises in employment law and is also a working mum. Sarah Calderwood has assisted with this answer.