Volunteers and their dogs will sleep rough for the night to try and save a drop-in centre from closure after it held a pop-up vets clinic for the dogs of homeless people.
Around a dozen people decided on the fundraising idea after the West London Mission centre allowed them to hold the Trusty Paws pop-up clinic for the dogs at the venue.
The event is due to take place at Manchester Square Garden, in Marylebone, near the centre itself.
Veterinary students and sponsors teamed up to pay for the clinic's medical supplies, including free vaccines, worming and flea treatments, clothes, food and microchipping.
In the unique event, participants are hoping to contribute to the £30,000 needed to keep the West London Mission Day Centre open after its funding was slashed in recent cutbacks.
Owner of Uptown Dogs Jamie Palmer will be taking part in the sleep out and will bring Sugar the Italian spinoni with him for the night.
He said: “Homelessness is a very complicated issue both for the homeless and the people trying to help them.
"When you factor in taking care of a dog under those circumstances too, the Trusty Paws clinic is a real life line and ultimately the catalyst for real changes in the lives of both the homeless and their dogs."
Sugar has her own Just Giving donations page as a way to thank the West London Mission for allowing Trusty Paws to set up in the day centre and has since raised nearly £900 for the cause.
A spokesman for West London Mission said: "The Sleep Out does not aim to replicate homelessness.
"But it will provide an inkling of what the clients of the Day Centre experience day in day out and the repercussions that this can have on you the following day."
A marquee will be set up on the evening to provide hot refreshments and entertainment for participants before they bed down for the night.
The following morning, volunteers have the opportunity to take a look around the centre and have a hot breakfast in the canteen.
Last year, a similar event for the centre saw 50 participants raise £24 000.
The West London Day Centre provides daily basic services for the homeless such as hot breakfasts, showers and laundry.
Big Sleep Out takes place October 16.