In the case of performance, dismissal should be the final step and taken only if your performance has not improved after warnings.
Your employer should have made the required standards of performance known to you, offered you training if appropriate, and provided you with sufficient opportunity to improve.
Employers should not apply unreasonable standards. Company standards should be achievable by the majority of employees doing the job. Where the employee is disabled, employers also need to have considered what reasonable adjustments could be made and have taken steps to avoid needing to make a dismissal.
If your employer, in dismissing you for capability, has not applied the same stringent performance standards to others, consider why. If you have been treated less favourably than others due to a protected characteristic (e.g. your race or gender) you may be able to complain of discrimination.
If you have been treated less favourably because of your lawful trade union activities, you may be able to claim automatically unfair dismissal and trade union victimisation. If so, you may qualify for interim relief. Speak to your rep urgently. The deadline for a tribunal claim for interim relief is extremely short. Your claim must be issued to the tribunal in just seven days. Acas early conciliation does not apply to claims for interim relief.
For guidance on bringing a claim, see workSMART's Enforcing Your Rights section.