As part of the move, announced on Tuesday at midday, no horse will be permitted to declare more than once at the meeting, whether subsequently declared a non-runner or not, unless eliminated from a race or declared as a reserve.
This change applies to all races except the five non-novice Grade 1 contests: Champion Hurdle, Champion Chase, Ryanair Chase, Stayers' Hurdle and Gold Cup.
All handicap races at the meeting will have two reserve slots, which will have a deadline of 13:00 the previous day, as is the case for races such as the Randox Health Grand National and Coral Scottish National.
Betfair spokesman Barry Orr said: "This is an extremely positive move for racing fans and the switch to 48-hour declarations is a decision that must be applauded.
"It will further enhance the build up to the greatest Festival in the world, with the previous situation of not knowing the final fields until 11:00 the day before being a hindrance rather than a help in this regard.
"The 'reserves' initiative and the prevention of double declarations are also very sensible and it's hard to see any downsides to this whatsoever. I'm sure punters will be fancying their chances even more of winning the annual battle with the bookmakers and will be putting the extra day of study form to good use."
The BHA said it will liaise closely with the rest of the racing industry to monitor the changes as they come into effect, particularly in relation to horses being declared for more than one of the championship races, which will be kept under review annually.
Richard Wayman, Chief Operating Officer at the BHA, said: "We have taken on board feedback from racing fans, the betting public and the media, and we're pleased to introduce 48-hour declarations for all races at the Festival, which we believe will have a positive impact on the promotion of the event and be welcomed by the sport's various customer groups.
"We appreciate that, in some circumstances, this could create an extra challenge for trainers but we want to ensure that the sport is in a position where it can make the very most of what are such an important four days in the British racing calendar."
All races at Cheltenham Festival will declare at 48 hour stage to provide greater early clarity for racing fans, bettors, media & bookmakers pic.twitter.com/rxmgy3ZQSf— BHA Press Office (@BHAPressOffice) September 12, 2017