This page explains the requirements for the employer's letter that you send to support your application to the highly skilled worker category (Tier 1 General) under the points-based system to work in the United Kingdom.
Full details of all requirements are in the immigration rules at paragraphs 245A-G of part 6A, and Appendix A, which you can find on the right of this page. You must read the policy guidance before you apply under this category. You can download the guidance and application form from the Applying section.
You must send a letter on company headed paper from your current and/or previous employer(s) which clearly shows: