The law says that a lack of capability is one of the potentially fair reasons for dismissing someone. The allegation, therefore, should be taken very seriously.
Capability in law is assessed by reference to 'skill', 'aptitude', 'health' or any other 'physical' or 'mental' quality. Your employer must state the reason and give evidence as to why it is believed that you are not capable of doing your job. The employer should also give you time to improve and provide necessary training to help you meet the required standards.
If you have recently joined your organisation, have taken on new responsibilities, or have been promoted, it may be that these circumstances have given rise to your employer's perception. If there is some substance to this view, you may wish to explore, with your employer, opportunities for further development or training that will help you meet the demands of the job. You may request that your employer considers letting you do an alternative job to which you may be better suited.
If the perceived lack of capability has anything to do with a disability, the law requires your employer to make 'reasonable adjustments' to enable you to perform. If your employer is not already aware of it, consider drawing your disability to its attention. Also, if you wish, you can make suggestions as to what reasonable adjustments would help you fulfil your duties.
Where you are accused of failing to meet performance standards and are at risk of being issued with a formal ‘performance warning’ or other disciplinary sanction, the Acas Code of Practice on Discipline and Grievance Procedures applies. Your employer must follow the Code, or else risk extra compensation being awarded if you are successful in a later employment tribunal claim.