I'm currently applying for a PhD but have just found out that Imperial College London has a policy not to accept students unless they have an Msc or MRes.

No other University has said this but does not having a masters significantly reduce your chances of getting a place?

It does depend on subject area. A masters degree is often required of most international students. In the arts and humanities it is more common for a Masters degree to be a requirement.

As far as we are aware Imperial College does not have a policy that all PhD students must have a masters degree. What subject area were you applying for?

I don't have a Masters yet was recently told I passed the first stage of selection for a PhD at Imperial. I don't know where you got the idea that you need a masters. It's untrue.

Having typed that, I just realised your point. The PhD I applied for is a joint 4-year MSc/PhD (doh). The webpage for these 4-year PhD's state they'll only waiver the masters year if you already have a masters. However, there are separate 3-year PhD's being advertised at Imperial which have no such requirement for a masters degree. There are some right here on findaphd.com.

The subject area I applied for was ecology. I've found no mention of a masters requirement on the imperial website but the supervisor of one of the projects I was interested in told me it was. I'm confused!

It may well be the policy in that department - ecology is one of the most competitive subject areas for PhDs as there are too many students chasing too few studentships.