A number of industry executives, including Ian King, the CEO of nuclear submarine builder BAE Systems, were in the audience to hear Fallon's warning.
"For 46 years our deterrent has been used every day and night to keep this country safe. If we are to continue that continuity, that continuous at-sea deterrence, there is no room in the program for delay," he said.
A BAE spokesperson replied that "BAE Systems is proud to design, build and commission the submarines which enable the UK to maintain a position of continuous at-sea deterrence. We are committed to working with the government and the broader submarine enterprise to maximize value for money whilst delivering leading capabilities on this important program."
Fallon's warning follows remarks made Oct. 14 by the MoD's top civil servant, Jon Thompson, that Successor was the program that most kept him awake at night.
"It's the single biggest future financial risk we face. The project is a monster ... it's a significant element of the overall equipment plan ... it most keeps me awake at night," the permanent undersecretary told a parliamentary public accounts committee hearing.
Thomson acknowledged getting accurate program costs would be difficult.
"It's an incredibly complicated area to estimate future costs but we will make them," he said.
The first new submarine to replace the existing Vanguard-class of boats is planned to enter service around 2028.
Fallon's reference to the Astute program having "slipped a little" is something of an understatement. The third of seven Astute-class boats is now undergoing sea trials but the program is years behind schedule and significantly over cost.
"Spread over the 30-year life of the new boats it represents an annual insurance premium of only 0.13 percent of total government spending, but we get a lot out from what we put in," Fallon said.
The government is expected to win parliamentary approval for the Successor project even though there is strong opposition from the Scottish National Party and parts of the Labour Party.
Maria Eagles, the shadow defense secretary, told the audience at the House of Commons that Labour Party policy was for a credible, independent nuclear deterrent.
That's at odds, though, with new party leader Jeremy Corbyn , a long-time supporter of the anti-nuclear movement in the UK, who is pushing for Successor to be canceled.
Andrew Chuter is the United Kingdom correspondent for Defense News.