Could just be something simple and harmless like slime mould, or it could be either Turf thatch fungal mycelium, Red Thread or possibly Snow Mould, but that usually causes dying grass which turns brown and then often pale pink fungal growths can be seen rather than white. A photograph would be useful.

I have the same on my lawn White mould and straw coloured dead patches, I've been told that it is fusarium patch. All my neighbours have it too. I'm hoping it will re grow in the spring.

Popedot, Fusarium patch is the same as Snow Mould, which is a name now more commonly used to describe this fungal infection. Treatment is essential or you will end up with no lawn at all. Spike all over frequently, do not walk on the lawn when it is frozen, covered in snow or very wet, do not feed during autumn or winter, carry out standard lawn feeding and weeding procedures during the growing season, continuing to spike regularly. If the condition is widespread, treatment with a fungicide watered on will be necessary.

Thanks Bamboo for your advice i certainly will try spiking the area, every day it is spreading even more. My neighbours dont seem too bothered by it, but i have put so much effort into my lawn this year to make it look healthy. I did actually feed my lawn 5 weeks ago with Evergreen Autumn feed do you think this could of triggered it, i think we've only had one frost attack here (Reading Berkshire) which was only very light. I'll be devastated if my lawn doesnt recover.

Let's be clear about one thing - Snow Mould is the commonest fungal infection in lawns in the UK, and it occurs most often from autumn through to winter, and is often unrelated to snow. I'm extremely irritated that the RHS has stopped calling it fusarium patch disease, because now everyone thinks you only get 'snow mould' if its been snowing, which absolutely isn't true. The connection with snow is simply that, if there has been snow and the grass has been walked on while its present, it will increase the chances of fusarium patch.
As for your autumn feed triggering it, unlikely - if you've not kept it well maintained by aerating, scarifying and feeding, and you've had drought followed by lots of wet, or even just lots of wet, then that's more likely the cause, especially if the lawn has been walked on frequently when very wet, causing further compaction. Benlate used to be the recommended fungicide to water on - but I have a feeling its now been withdrawn, though no doubt there is a substitute.