Labour would give workers the right to choose their working hours from their first day in a new job, the party's equalities spokeswoman is to say.
Employers would be required to allow their employees to work flexibly from day one, Dawn Butler will tell the party's Women's Conference later.
This would open up more roles to women who care for loved ones, helping to close the gender pay gap, she will say.
Workers can currently request flexible working after 26 weeks in post.
This can include features such as job-sharing, working from home, part-time, annualised or compressed hours or flexi-time.
However, employers can reject requests provided they do so in "in a reasonable manner".
Labour says that - if elected - it would create a "presumption in favour of flexible working", requiring employers to design the potential for people to work flexibly when creating roles.
"This change to the law is essential to closing the gender pay gap and dismantling the structural barriers that hold women back from promotion and progression," Ms Butler will say.
Highlighting government statistics on "sandwich carers" - those who care both for children and elderly or disabled relatives - Labour says women account for 68% of those providing 20 or more hours of free adult care per week.
"Women do the vast majority of unpaid care, but this must not be a barrier to women in work," Ms Butler will tell the conference in Telford.
"Under Labour's plans, no woman will be shut out of the workplace because they're a mum or they care for a parent or a disabled loved one, or both."
The plans would place an onus on employers who wanted to reject applications to prove roles were unsuitable, in line with conditions set out in new statutory guidance.
Under current laws, employers can reject applications for various reasons such as that they will lead to damaging extra costs, could affect quality and performance, or work cannot be reorganised among staff.
The government took steps to promote flexible working when launching its drive to force large employers to publish their gender pay gaps.
In October 2017, Prime Minister Theresa May called on employers to advertise all jobs as flexible from day one, unless there were "solid reasons" against doing so.
A Flexible Working Task Force, chaired by the Chartered Institute for Personnel and Development and including policymakers, employer groups and unions, began work in March last year.