The building society says the subsidiary will close to business from June 24.
Astra was launched in 2008. The mutual has blamed the downgrade by Moody’s in April for the move, informing advisers that a lower credit rating has made it tougher to secure funds.
N&P says that when the market recovers, it plans to re-open Astra.
N&P business manager Scott Taylor-Barr wrote: “We have always lent sensibly and run our business prudently within the resources available to us but the recent downgrading from Moodys is having an impact on wholesale funding and Astra’s ability to lend.
“The increased cost of funding has cut our profits and limited our ability to expand our lending.
“Although Fitch has subsequently confirmed N&P’s long-term credit rating, we cannot see the wholesale funding market position changing soon and have decided to focus on providing mortgages through N&P retail channels by retail deposits.
“N&P will however remain open for Commercial Mortgage business from intermediaries.”