Thanks @Chita. My daughter is a seamstress and she's coming round on Sunday. So I have nearly four days to lose a bit of weight and she'll see what can, if anything, be done. If it's hopeless I will go back to the shop on Monday to see about an exchange. It's a blue dress, (chiffon) from under the bust downwards, and cream above, with a cream bolero to match. I'll be gutted if I can't wear it.@Happy Lady
If you think you can sit in it for the ceremony and lunch/dinner then do that but take something to change into afterwards.
Is it a dress? Or skirt and top?
Plain or patterned?
How long is it?
If there's a slit at the back and the skirt is long enough, you could have a slit put in the front by a seamstress/alteration service.
Like making a "false hem" on someting, a slit can be made.
If it's a skirt and top, can a different skirt be used?
If its a frock and in plain fabric, or even patterned if you can find a suitable piece of fabric to go with it the dress can be unpicked at each side seam and two strips/panels created to widen.
It can be made to look like a feature as if the panels were always there. But will be tricky getting a match.
If it's never been worn, I would take it back to the shop you got it from and ask them to help.
Maybe exchange the whole outfit or part of it.
People in the industry are used to dealing with crises and can often fix things we think are just beyond solving.
Do it now while you have time.
The food I've been ignoring since the weekend is helping. I've lost 2lb. Time to lose another 3 or 4 at this rate, hopefully.