Most research ethics committees only permit expedited review in exceptional and clearly justified cases - otherwise everyone would ask for it, because everyone is anxious to get on with their research. So if you think your proposal might be eligible for expedited review, consider the following:
- Is it really necessary to have expedited review?
- Are there any external drivers which are outside your control (like pressure from a funder, or access to a sample) which mean that you have to get ethics clearance within a short timescale? For example, is your fieldwork linked to a particular event or period that is outside your control (such as school terms, or the football season)?
- Does your project have an exceptionally short timescale?
Check with your research ethics committee how soon you can apply. Most committees will not accept proposals for research that is not definitely going ahead (e.g. after you have written the proposal, but before the work is funded), although some may - or they may make an exception. However, many research ethics committees will review applications after the work is confirmed, but before the contract is signed (recognising that some funders can be very slow to produce contracts, although you may have a letter or email confirming your funding). Check what your committee will accept, and apply as early as you can.
Just having left it late, or not realising that you had to get ethics approval, may only very reluctantly be accepted as justifications for expedited review - and you run the risk that these reasons will not be accepted at all.
Remember too that most ethics committees will not issue approvals retrospectively - that is, for work that you have already done.