Can I use current passport while applying for new one online

[talksthetorque]

Need to organise a new passport, it has 4 months left.
I have to go away with work in the meantime.
I'm thinking of applying online, my question is

Can I use my existing passport while I am applying for my new one?

The details on the website are a bit sketchy. Government cutbacks and all that.

Or Do I have to send my old one back in?

[talksthetorque]

Thanks people. Currently waiting for an answer on whether work wants to pay for speedy service or put up with me not being able to travel abroad (told them 3 weeks, just in case)