Spratt's initial deal expired after Saturday's thumping 5-0 win over Bamber Bridge, but boss Darren Kelly has been able to extend it for a further month.
Delighted boss Kelly said: "I'm really pleased that we've been able to extend Harry's loan deal from York City.
"Harry has been a breath of fresh air since he came into the club, he's put in some really strong performances and will only get better too.
"He's a player I have known and admired for a while now, so I am delighted to be keeping him with us."
Boss Kelly is also looking to strengthen his squad further ahead of their trip to Warrington on Saturday.
"We're looking to do some business ahead of the weekend and hopefully we can get that over the line," added the Boro boss.