An employee seconded to a host company may become that company's employee if the secondment is open-ended and the host company takes over day-to-day control of the employee.

An employee was seconded 'permanently' and it was expected she would remain so until her retirement and would not return to the original employer. She confirmed that she was relinquishing her role with the original employer and her salary and conditions of service were determined by the host company. In these unusual circumstances, she was held to have become the employee of the host company, notwithstanding the original employer's agreement to provide a comparable post should the arrangement come to an end. 

Employers should bear in mind that simply calling an arrangement a secondment will not be conclusive if, in reality, there is no intention for the secondee to return and the host company has assumed day-to-day control. (Fitton v City of Edinburgh Council, EAT)