Victorian school funding.

Federal Labor has promised to backdate any school funding withheld from Victorian students if it wins the federal election.

As the stoush over school funding intensifies, Deputy Opposition Leader Tanya Plibersek has written to Premier Daniel Andrews to assure him funding for Victorian schools would increase under a federal Labor government.

"Unlike the Morrison Liberal government, I would not hold school funding for students across public, Catholic and independent schools to ransom," she wrote in a letter on Tuesday.

It is looking increasingly unlikely that Victoria will sign up to a long-term school funding agreement with the federal government before the May federal election.

Mr Morrison says he's received legal advice that says the Commonwealth cannot make payments to Victorian schools without a signed national school reform agreement.

He's accused the Victorian government of using “non-state school parents and students as hostages” by holding out on an agreement.

Catholic and independent schools are also concerned they could miss out on vital funding unless an agreement is signed this month.

But Mr Andrews says his advice confirms that money should not stop flowing to any Victorian school, regardless of whether an agreement is in place.

The deadline to reach a school funding agreement passed on Friday, putting federal funding for the 2019 school year into limbo.

Ms Plibersek said if Victoria did not reach an agreement with the Commonwealth before the federal election, a Labor government would ensure the funding was paid and backdated to the start of the 2019 school year.

"Federal Labor is committed to fair and equitable funding for every Australian school and to delivering the resources and reforms necessary to ensure every child has the opportunity of a great education, no matter where they live or which school they attend," she wrote.

She said that by 2022, Labor would increase funding for state schools to 22.2 per cent of the Schooling Resource Standard – the bedrock of the Gonski review which measures how much funding every school needs to support students.