It Can be done,

Old fire exits can be blocked up, i have just done it to a building myself it required a full risk assessment of the building, by blocking off 2 fire exits at the top of the building we extended the minimum traval distance single travel to 58 meters, this is a big no, far too long compared to the recomendations of part B building regs.

The area was a conversion of a mezanine floored store area,to an office area. to comply we had to get building control and the fire brigade in.

We designed and built a fully protected escape corridor around the perimiter which made the longest 2 way escape traval distance 46 metres (to long) which we offset by adding a full L1 alarm system.

To just close an exit could be risky there is alot to think about, also it should be written in the lease if an ajoining business requires transit through in the event of an emergency. If the adjoining business has just locked of an exit route they could be in breach of fire law.

Gasmonkey, there are lots of reasons for having fire exits in the first place and other reasons for them becomig obsolete.

I am assuming this is a fire exit and not a fire door inside the premises designed to prevent fire spread?

Psyco's story is very descriptive of the lengths he went to ensure his fire escape route complied but his premises is different to yours.

The only way to be sure is to have a proper fire risk assessment carried out by someone who understands the requirements and is competent.

It would be foolish for the likes of myself to offer guidance without actually seeing your location.

I hope this helps, not being negative but means of escape is so important, as are all other requirements and fire precautions.

Good luck.