Best to get a damp expert in, but it looks like it could be efflorescence? Also, in older properties, if the wrong type of plaster is used, or even if the paint is too thick, then the walls can't breathe. You might want to scrape the paint off and let the walls breathe for a bit. My friend had the same problem in her period property and called in a damp expert. They said it wasn't damp (we are all too quick to blame damp these days) nut that the previous owners had painted the walls with too thick Farrow & Ball paint and there were no vents in the double glazed windows.

Have asked the expert sitting to me who says it is damp and the salts coming out.

I also think it is water damage- it could be rising but equally it could be water that has travelled from elsewhere.
Is there a soil pipe nearby? Or a drain just outside? Or a flat room above?

Hi all. Thank you for your comments. I will get scraping the wall to see what is behind it! :)

Hi this definitely looks like damp. Be wary of getting 'damp specialists' out to assess it. They will undoubtedly suggest all the plaster is hacked off up to one metre, the wall injected and then replastered. You may find you have to do this in the end up but it is worth taking time to investigate. Do you know if the wall has previously been injected? If so you may find that the damp proof course has been breached. We had this problem in a previous home. If you take the skirting board off, see how far down the plaster goes. If it is down to the subfloor the water could be wicking up the wall. Damp stain and blocker paints are a waste of your time and money btw ,they only mask the problem temporarily.

Looks like efflorescence to me. We had it in the kitchen and it looked exactly like that. If there is white dust/salt behind the paint which looks like mould but has no real odour. We scraped the paint off, bought a salt neutralising solution online, painted it on and then left a week. We had to do it twice because we'd left it so long it had spread up the wall.

Thanks Laura. Did the salt neutralising solution fix the problem?

We have exactly this problem. In fact I had to do a double take on your photo thinking it was my wall. My problem is also on a spur following an extension in the dim and distant past and there has never been a problem there before. Watching with interest.

We hacked off the plaster and re-plastered and painted. That fixed the problem for all of 2 months...

Did you re-plasterboard when you hacked the plaster off or just re-skim? Thinking hack the plaster off to brick and putting plasterboard on and then skimming see if that helps

Yes, it did work Mike.

Hacking off may not work. The problem is with the bricks, not the plaster.

It's the salts coming out of the bricks so this is what you need to sort. Simply hacking off the plaster won't work.