Legislation will be introduced to change the "clearly outdated" rules and allow the attorney general to take maternity leave.
Suella Braverman announced in November that she was expecting her second child early this year - and she is believed to be the most senior woman at cabinet level to have a baby while in office.
But under the current rules, she would have to resign in order to take time off.
Boris Johnson's spokesman said the rules were "clearly outdated and need fixing", hence the legislation being introduced.
It will provide for six months' paid leave, with the bill considered by parliament next week.
Mrs Braverman will temporarily be replaced in her role until her maternity leave is over, it is understood.
"At the moment women are offered the choice between either resigning from their position or taking time off to recover from childbirth, which is simply not acceptable in the modern times," the prime minister's spokesman said.
He added that the legislation "would apply to anybody who holds a government position, whether that's a cabinet minister, a junior minister or anybody who holds a government position to allow them to take six months' full paid maternity leave".
The PM outlined details of the changes in a written statement published on Thursday.
He wrote: "Changes that I made to The Ministerial Code on becoming prime minister set out provision for junior ministers to be able to take maternity leave.
"However, this work-around relies on another minister taking on additional responsibilities and cannot be used for Secretary of State or individual offices, such as the Law Officers or the Lord Chancellor.
"Until now, the limits on the number of salaries that can be paid overall, and for individual offices has left the government with limited flexibility to appoint cover should a minister want to go on maternity leave.
"In the absence of that flexibility, a senior minister wishing to go on maternity leave would likely need to resign from the government.
"The bill creates a designation of 'Minister on Leave' which provides for ministers to take maternity leave. This will also apply to certain Opposition post holders too.
"Ministers on leave will remain part of the government and be able to be briefed on matters and kept in touch with work, but will not be responsible for exercising the functions of the office from which they are on leave."
Labour's shadow Cabinet Office minister Rachel Reeves said the move was a "small but significant step forward for women's rights in parliament".
But she was critical of the lack of provision for paternity, adoption or shared parental leave, saying this meant the legislation was "already out of date".
"The attorney general will be the first cabinet minister in UK history to take maternity leave with proper pay and cover," Ms Reeves said.
"It is quite unbelievable that women cabinet ministers throughout history have faced resignation or demotion when choosing to have children.
"Working women should not be forced to choose between having children and their career - whether that be a cabinet minister or any other working mum.
"But this bill is already out of date, with no provisions for paternity, adoption or shared parental leave.
"Labour intends to hold the government to their word to work cross-party to introduce comprehensive legislation in the coming months to right this injustice."