Nelson v Kingston Cables Distributors Ltd, EAT case no 662/99 on 2nd May 2000, (not reported but full text of the judgment is on the EAT website).

an employer who indicates to an employee who is absent on maternity leave that the job she will be required to do on her return will be significantly different from what she had been doing previously commits an "anticipatory breach of contract" with the effect that the employee is entitled to treat herself as constructively dismissed immediately.

For relevant general notes see Changes to terms of employment , Constructive dismissal/a general note and/or Maternity/maternity leave/return to work (general) .