The honeycomb biscuits topped with milk chocolate hit stores this week and have been billed as Fox's new flagship range.
They will be backed by a £3m marketing campaign starring Vinnie the panda in May a record marketing spend for the company.
They come in praline, caramel and plain honeycomb flavours, which Northern claimed would offer consumers "a better class of naughtiness" at an everyday price.
At £1.39, they cost 30p less than Burton's range of Cadbury biscuits [BrandView.co.uk].
"There are no other biscuits out there that really nail the everyday indulgence message," said Rachel Moffatt, brand sector director. "Our rivals have underestimated the potential of Fox's in the past but they must accept that we have been steadily investing these past two years.
"United Biscuits and Burton's will have an eye on us. Ambers will make indulgent flavours like praline completely accessible and affordable to biscuit lovers everywhere."
Over the past year, sales of Fox's Biscuits have increased 5% to £161.8m [Nielsen 22 January 2011].
This week, Greencore was poised to re-enter the battle for control of Northern Foods following reported talks with potential private equity backers.