Overgrown Philadelphus

I have inherited a philadelphus that is trying to be a tree - in other words, over 12ft tall. It hasn't been pruned in over 12 years, and is now very leggy. I spent the weekend removing an infestation of ivy from around it, which has made it even more obvious how desperately it needs pruning.
If I pruned it now, to say half its current height, am I a) likely to kill it b) going to cancel out any flowers this summer? Help and advice please.

 

Hello Ihateslugs and welcome to Gardeners Corner.
If you prune your Philadelphus now you will lose this years flowers.
Philadelphus is a lovely shrub and well worth saving so this is what I would do.
Wait until your Philadelphus has flowered - around June.
As soon as it has flowered cut about a third of the stems back nearly to the ground. This will give you good growth from the base.
On the remaining stems cut them back by about a 1/3 to a half. This will give you good flowering next year.
Next year cut a different third to the ground and so on.
That way you will keep the shrub under control and at its best.

 

Yes I would agree - thats good advice from Alice.

There's a bit of extra info here http://www.gardenseeker.com/pruning/pruning_philadelpus.htm