Brits have been warned the London Ambulance Service is "extremely busy" and will be focusing on the most sick as it tackles the situation.
The ambulance service has said it has put in place a "Business Continuity Incident" in order to cope.
It comes with around 10,000 ambulance workers due to walk out in a strike that will run from midday on Wednesday until midnight.
Members of UNISON will be taking national industrial action and will include “members who staff ambulances and other response vehicles, but not the call handling teams in our 999 or 111 control centres.”
London Ambulance Service tweeted: "Our staff are working hard, but our 999 and 111 services remain extremely busy. As part of our escalation plans, we've declared a Business Continuity Incident to help us manage the situation.
"We are prioritising our sickest and most severely injured patients."
It also asked people to only call 999 for a "serious medical emergency".
It continued: "You can help us by ONLY calling 999 if it is a serious medical emergency (which includes, but not limited to, unconsciousness, chest pain, difficulty breathing, severe loss of blood and choking).
"Using 111, Online http://111.nhs.uk if you need help fast but it's not a life threatening emergency. Making your own way to hospital if it's safe and possible for you to do so."
London Ambulance Service said that due to the strike action people whose conditions are not life-threatening are "unlikely" to get an ambulance.
It stated: "On the day of industrial action, there will be fewer ambulances on the roads, with the NHS prioritising those with life-threatening needs. As a result patients whose conditions are not life-threatening are unlikely to get an ambulance on industrial action days.
"Where the situation is not life-threatening, alternative support will be available through NHS111 online or through NHS 111, and where possible, it is advised that you arrange alternative transport to hospital.
"Like health and social care services across the city, London Ambulance Service is currently extraordinarily busy, with record numbers of urgent and emergency calls coming in and high numbers of patients waiting for an ambulance."
The impact of the strike is also likely to be felt in the days following the strike.
"While the industrial action will last for a 12-hour period, LAS expects the impact on services to be felt across the whole day and into the following days as it works to help as many people as possible," added the London Ambulance Service.
