The Military Wives choir are expected to have sung their way to the top of the charts.
Their single, Wherever You Are, had sold 319,000 copies by midnight on Tuesday, outselling the rest of the Top 20 combined.
The Christmas number one will be announced at 3pm on Christmas Day, during the festive edition of BBC1's Top Of The Pops.
Gareth Malone formed the choir for BBC2 programme The Choir, which climaxed with the group performing at the Festival of Remembrance at the Royal Albert Hall.
After a mid-week sales update, Official Charts Company managing director Martin Talbot said: "This truly is an extraordinary result for Gareth and his choir - easily now the fastest selling record for years.
"They are extending their lead day by day and breaking records as they go."
Proceeds from the single are being donated to the Royal British Legion and the Soldiers, Sailors, Airmen and Families Association. The choir received a boost when the Government said it would waive VAT on sales of the single.
The Treasury will give the money to charity in lieu of the tax taken from sales, having last month agreed to a similar gesture for The X Factor's charity single.
Some of the choir members will be working at Tyspane Care Centre in Braunton, north Devon, when they find if their single has hit the top spot.
By Wednesday, X Factor winners Little Mix had the second spot in the charts with their single Cannonball. Lou Monte's Dominick The Donkey was at number three, and fourth place went to online act Alex Day, who wrote and recorded his track Forever Yours in just 24 hours.