Blocking fireplace opening - air vent or not?.

I am starting to renovate/modernise a Victorian terraced property and part of the renovation is removing the old fireplaces in 2 bedrooms and one downstairs room, all on chimney breast. There will be no working fireplaces in these rooms

I need to either brick or plasterboard the opening and finish with a plaster skim.

Conflicting advice from 2 different sources one suggesting required venting the other adamant not necessary - neither able to give reasons.

My question is it really necessary to include either an air brick or vent in the sealed opening and if so why?

Yes it is necessary. Fit a vented rain cap to the top to stop water entering via the pot & a vent at the bottom to allow a natural updraft & air circulation.

No air circulation could allow condensation to form on the inside of the chimney & moisture may gradually seep down from an uncapped pot or through the exposed stack; or both.

Much more important, is that condensation will react with the flue gas deposits and form a solution which chemically attacks the cement in the mortar - turing it to powder, and then the bricks start to drop or move

Thank you both - now clear on the need for ventilation - already have enough problems with damp (penetrating and possible rising) so will be following the advice to get a vented raincap fitted and fit a small vent in the closed fireplace opening.