An undertaking is a promise made by a solicitor in his capacity as a solicitor that he will carry out or refrain from carrying out an act in circumstances where the recipient of the promise reasonably places reliance on it. The undertaking need not be in writing.
In Briggs v the SRA  EWHC, the High Court said that solicitors’ undertakings are the bedrock of the conveyancing system. Purchasers and mortgagees need to have complete confidence that money put in the hands of solicitors in the course of a conveyancing transaction will be safeguarded by undertakings and without that confidence the system of conveyancing would break down.
A breach of an undertaking can lead to disciplinary proceedings and would normally result in a rebuke or a fine by the SRA under its internal disciplinary powers (in less serious cases) or a fine by the Solicitors Disciplinary Tribunal (in more serious cases).
Disciplinary proceedings do not normally give the recipient of the undertaking a remedy as the SRA and the SDT do not have direct power to require a solicitor to comply with an undertaking. A breach of an undertaking can be enforced by court proceedings. A civil claim can be made for specific performance or compensation. An action can also be taken in the High Court under its inherent jurisdiction over solicitors to compel the solicitor to comply with the undertaking or to pay compensation is that is no longer possible.