Door frame without a door?

Hello,

Someone has told me that it is illegal to have a door frame without a door, is this true?

In the house I'm due to let there are two entrances into the lounge from the kitchen, one has a door the other one doesn't, and has never had one and I expect it would be quite hard to put one in as the stairs slightly overlap one side and it's small space between the front door and built in kitchen furniture the other.

I suppose if it is illegal the only thing I can do is block it up and use the other door?

Property Type: 3 Bed Terraced House, to be let to 1 tenant (family)

Thanks

Matt