Crystal Palace Ladies will play in the Women's Championship after Doncaster Rovers Belles joined Sheffield FC in withdrawing from the league.
The Belles won tier two - then called Women's Super League Two - in May, but need to leave the revamped division to "ensure their continued viability".
Both they and Sheffield FC, who also dropped out for financial reasons, will now play in the third tier.
Palace will now fill one of the vacated spots, having met the licence criteria.
They will be alongside Manchester United in the new second tier, after United were one of five sides to earn Championship licences in May as part of the Football Association's restructuring of the women's leagues in England.
Palace expressed their "overwhelming disappointment" when missing out at the time, but their bid met the requirements and therefore they were deemed next in line to come up from the third tier, once existing tier-two sides Sheffield FC and the Belles pulled out.
Palace Ladies chairman Richard Spokes told BBC Sport: "While there is a lot to do in a short space of time, we have a team, fully supported by Crystal Palace FC, which has proved its viability over many seasons and is ready for the challenges ahead.
"During what has been a trying few weeks, the support from our fans and Crystal Palace FC, actively led by Palace chairman Steve Parish, has been outstanding and a real boost."
The changes provisionally leave 11 teams in the new second tier, with 11 teams also playing in the new, full-time top flight - the Women's Super League.
However, the final number of teams to play in tier two in 2017-18 is still yet to be confirmed, with Sunderland, Blackburn Rovers and one further unnamed club having appealed against their omissions.
BBC Sport understands that one of those appeals remains unresolved, but if the current league arrangements remain in place then the FA will look to balance out the top two divisions with even numbers of teams next summer through promotion and relegation, with a two-up, one-down system being considered.
|The 2018-19 Women's Championship|
|Aston Villa Ladies||London Bees|
|Charlton Athletic Women*||Manchester United Women*|
|Crystal Palace Ladies*||Millwall Lionesses|
|Durham Women||Sheffield United Ladies*|
|Leicester City Women*||Tottenham Hotspur Ladies|
|Lewes FC Women*||(* - joined by licence application this year)|
Why is the Belles' withdrawal so significant?
Only Arsenal have appeared in more Women's FA Cup finals than the Belles' 13, with the Yorkshire club winning six times between 1983 and 1994.
They were one of the WSL's eight founder members in 2011 but were controversially relegated prior to 2014 when it expanded to include WSL 2, with Manchester City replacing them - a move which ultimately "saved" the Belles from folding.
Doncaster won WSL 2 by 10 points in May but did not go up because traditional promotion and relegation had been scrapped for 2017-18, replaced by a licence application process.
The Belles did not apply to move up to the new, professional top tier and then initially received a tier-two licence for 2018-19 in December.
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The revamped second tier requires teams to provide contact time for players of eight hours per week plus matches.
A Belles statement said: "We have worked closely with the FA and mutually agreed the change in leagues which will ensure the continued viability and re-growth of the club.
"Recent changes have allowed us to reflect and refocus. We are excited to see a wealth of young developing players, who have a great opportunity to hit their full potential under a fantastic coaching team."
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