I was diagnosed with asthma at a very young age and now due to the pandemic, at work im regarded as high risk for contracting the virus.now they require a doctors letter stating if i am fit to go back to work soon so please advise.

Asthma/Covid: Chronic Asthma is one of the conditions that makes having Covid more dangerous. You are at higher risk, and therefore should take more stringent precautions. Insist on a workspace well separated from your colleagues and if this is not possible, use a mask and ask those around you to use a mask. Don't use public amenities like lunchroom and coffee makers. Sanitize frequently, and watch for symptoms

Asthma: If the question is regarding how to get required letter for return to work, your GP will be best positioned to provide such a letter as being aware of details of your medical conditions.