The deal is expected to be finalised during the first quarter of the year and has been two-years in the making.
Keystone has been originating loans for UK Mortgages Limited (UKML) for the last two years with warehouse financing provided by National Australia Bank (NAB).
An announcement from UKML confirmed the Hops Hill No.1 PLC securitisation of buy-to-let mortgage loans originated by Keystone Property Finance.
The portfolio had reached almost £340m by 31 December 2020 and a transaction of up to £400m is now being marketed, to include loans currently in the pipeline and expected to be completed over the next three months, it said.
Roadshow meetings are being offered to investors, following which the book will be built and the transaction launched, subject to market conditions.
NAB and Santander are co-arrangers and are joined by Standard Chartered Bank as joint lead managers.
TwentyFour Asset Management is also supporting the securitisation.