Camelot said a single ticket scooped the top prize in the “Must Be Won” draw by matching all six numbers.
The claim will now go through the process of validation – including checks to ensure the terms and conditions are adhered to and the integrity of The National Lottery is protected.
Once the ticket has been validated and paid, the ticket-holder will then decide whether or not to go public, Camelot said.
Saturday’s winning Lotto numbers were 14, 15, 31, 41, 44, 45 and the bonus number was 20.
Andy Carter, senior winners’ adviser at The National Lottery, said: “We’re delighted that this special £20 million ‘Must Be Won’ Lotto jackpot has been claimed.
“Our focus is now on supporting the ticket-holder through the process and helping them start to enjoy their truly life-changing win.
“We’re also still searching for a missing £7.4 million Lotto jackpot winner from the draw on June 18, 2022.
“The missing winner bought their ticket in Wolverhampton and we’re renewing our appeal for Lotto players in the area to check their tickets to be in with a chance of claiming another incredible Lotto jackpot prize.”
Wednesday’s Lotto jackpot is an estimated £2 million.