Everyone is entitled to a private life and having an affair with a colleague is not normally a sacking offence. If you have been sacked just because of the affair, and have worked for your employer for more than a year, then you can complain of unfair dismissal to an Employment Tribunal.
It is unlawful for employers to treat women or men less favourably because of their sex. If only one of you is being asked to leave because of the affair, then you may also have grounds for a sex discrimination claim, regardless of how long you have worked for your employer.
If the relationship ended badly you should be careful not to let personal bad feelings spill over into your working relationship. Otherwise it is possible that you could find yourself being accused of sexual harassment or discrimination, particularly if you are in a position of seniority over your colleague.
Either way, your employer will have to follow a proper disciplinary procedure, giving you a right to appeal, before they are can dismiss you. Make sure to take legal advice on your situation, and talk to your workplace union representative if you have one.
