The South Ayrshire airport's sole passenger airline has confirmed it will reduce the number of weekly flights this winter from 16 to 11.
The number of routes served by the budget airline will also go down, from eight to five, with Fuerteventura, Malta and Rzeszów in Poland being lost.
They had been previously reduced from ten in 2017, and compares to 42 in 2013.
Flights to Tenerife will be reduced from four to one flight a week with Ryanair closing its base there in January .
Prestwick's other remaining flights will be four a week to Alicante, two to Faro, two to Lanzarote and two to Malaga.
An industry source said: "I'm pretty sure it is the lowest ever.
"It works out at approx 500 passengers per day during winter.
"They'll be 20,000 seats down compared to last winter."
Ryanair launched services from Prestwick 25 years ago with a twice-daily link to Dublin.
A Ryanair spokesman said: "As announced in July, due to the late delivery of up to 30 Boeing MAX aircraft, a number of loss-making Ryanair bases will be cut or closed this winter.
"We are working hard to minimise the impact on our customers and our people, and to save as many routes as possible, which will be served by flights from other bases."
The news coincided with the deadline for bids to buy the debt-laden airport.
Up to three tenders were due to be submitted by today to the Scottish Government, which bought the site six years ago to avert its closure.
It has since been loaned nearly £40 million by ministers.