The report in The Sun citing an unidentified royal source came five days after Middleton attended a concert celebrating the life of William's mother, Princess Diana, further fueling rumors that the two have reconciled.
William's Clarence House office has declined to comment on media reports that the two are back together.
William, second in line to the throne, broke up with Middleton in April as speculation swirled that the two were about to marry.
Middleton had been pursued by photographers outside her London home and nightclubs in scenes that drew comparisons to the coverage of Diana.
The Sun report prompted bookmaker Ladbrokes to announce it would no longer take bets on whom William will marry.
After the split, bookies initially put the odds on the couple marrying at 16 to 1. The odds eventually improved to 1 to 3 as more gamblers put their money on Middleton.
"We thought the pair had split for good," Ladbrokes spokesman Nick Weinberg said, adding the public "clearly had more faith than the bookies."