Can an employer be liable for harassment of an employee by colleagues because of the employee's religion or belief?

Yes. Under the Equality Act 2010, an employer will be liable for harassment by its employees in the course of their employment even if it was unaware of it at the time, unless the employer has taken all reasonable steps to prevent the employees from harassing others. As a minimum, to have a successful reasonable steps defence, an employer is likely to need to have a policy in place dealing with harassment related to religion or belief and to have taken effective measures to ensure that employees are aware of, and understand, the policy. This is likely to require some training of employees.

The definition of harassment under s.26 of the Equality Act 2010 is very wide. The unwanted conduct need only be "related to" religion or belief; it does not have to be directed at the victim's actual or perceived religion or belief. Further, the conduct may concern the religion or belief of a third party and/or be directed at a third party.