Hivac v Park Royal Scientific Instruments Ltd [1946] All ER 350, Court of Appeal (date in 1946 not clear).

in deciding whether an employee should be restrained from working in his spare time for an employer other than his main employer the courts will, apart from any contractual terms, carefully balance all relevant considerations including the nature of the work, the status of the employee and in particular whether the two employers are in the same type of business and are or are likely to be competitors.

For relevant general notes see Implied terms in employment contracts/a general introductory note .