A restart to the Scottish League 1 & 2 season has been approved from 20 March, but the SPFL board remains unconvinced that a 22-game season is realistic.
That is the aim of the 20 clubs after First Minister Nicola Sturgeon gave the go ahead to a return to play.
But the governing body is seeking assurances about what will happen if clubs fail to complete their fixtures.
"Only then will the board be able to consider the requests to push back the dates of the play-offs," it said.
The board had met at short notice to consider its response to the Scottish government's approval of a return of the leagues, which had been suspended in January, provided there is appropriate Covid-19 testing.
With some teams having played as few as eight of their planned 27 matches, there have been doubts about completing the season.
But the clubs want to reach 18 games, then split for a further four matches, up to and including 15 May - two weeks later than planned - and delay the start date of the Championship-League 1 and League 1-League 2 play-off matches from 5 May to 19 May.
"The board remains concerned regarding the realistic ability of League 1 and League 2 clubs to complete a 22-match season, including the introduction of a new 'split' in each division," the SPFL said.
"Such a fixture schedule would mean one club in each half of the 'split' sitting out one round of fixtures post-split, including the final day of the season.
"It would also severely constrain the flexibility of the SPFL to schedule fixtures in the event of future disruption from Covid-19 and risks part-time clubs being required to play Saturday-Tuesday-Thursday-Saturday.
"It is also guaranteed that a minimum of one League 1 or 2 club will reach round four of the Scottish Cup, creating further fixturing challenges."
The SPFL board "respects the right of League 1 and League 2 clubs to determine how many league matches they wish to play", but chairman Murdoch MacLennan said it "has severe reservations" about the practicality of part-time clubs completing the desired length of season.
"In the board's view, an 18-game season would have created far more flexibility," he said.
The clubs say that reducing the term to 22 matches would allow "a competitive end to the season" and that players "have made it clear" they want to play those games.
That would mean every team would have played the other twice, with two more home and two more away games to be decided by the league positions at that stage.
They point out that only the side who finish second bottom in the Championship would be impacted by delaying the play-offs.
"We received money to test and to allow us to keep playing and that is what we want to do, not just run down the clock to the end of the season," the clubs said in a statement.
'The players just need to be ready' - analysis
Former Scotland international James McFadden on Sportsound
The starting point has to be just to get back training and get the players as best prepared as possible. If they have to go back and play three games a week, the players need to be ready for it.
Nowadays, you get less and less time off, but the fact these guys have been doing nothing - and they can try to tick over as much as they want - you can't replicate training on your own.
You've got contracts to look at, promotions, relegations, lengthening the season would be difficult. It's going to be a big ask, but if you ask anyone involved, they'll just be desperate to get back.