Dear Garima,
First of all, is there a written HR manual in the company? If yes, you can refer to the article number/page number/section number (whichever applies).
If no, then you have to provide the employees with the scheduled timings.
Your letter can be as follows;
Dear Colleagues,
It has been observed by the management that the company's rules and regulations are not being followed by some/all employees.
Some of the areas where rules are not followed include attendance, lunch timings, ____, ____, etc.
As a professional organization, we would request all the employees of ________ to adhere to the company's rules and regulations.
For the benefit of employees, we are mentioning the common rules below:
1. Office Timings: _________________
2. Lunch Timings: ______________
3. ______
It is our sincere belief that the company's rules will be followed in accordance with good faith by all employees.
Henceforth, if employees are found to be neglecting or not conforming to the rules, strict action will be taken against the particular employee.
Yours sincerely,
HR Department