Chipping Barnet MP Theresa Villiers joined nursery staff at Downing Street to demand an increase in funding for nurseries which support many vulnerable children.
Ms Villiers was among a cross-party group of politicians to deliver a petition to Number 10 on Tuesday.
The petition, which had been signed by 2,000 school leaders, staff and educators, calls on the Chancellor to "take urgent action to provide adequate funding" for maintained nursery schools.
The campaigners say council-run nursery schools, many of which are in the most disadvantaged regions of England, need more support to address the impact of the Covid-19 pandemic on young children.
Protesters marched from Parliament Square to Whitehall, waving banners reading "our children need us" and "save our nursery schools", in the event organised by the National Association of Headteachers (NAHT).
Conservative MP Ms Villiers told the PA news agency she was representing maintained nursery schools in her constituency which have lost out on funding "over the past four or five years".
She said: "The Government has promised a new financial settlement to get them on a firm footing for the future, but it just hasn’t happened.
"I am always prepared to stand up for my constituency and sometimes sadly that means criticising the Government.
"They have promised a solution and I am confident that we will get one. But I feel it’s my job to keep up the pressure until that happens.
"So I’m speaking up here, it’s maybe not the normal event that I would attend, but I’m determined to save these schools."
She added that if the Chancellor does not "step up and give them a fair funding settlement" it is "inevitable" that "many more" nurseries will be forced to close.
Ms Villiers was joined at the protest by Tory colleague Jeremy Wright, as well as Labour MPs Lisa Nandy, John Cryer, Apsana Begum, John McDonnell, Tulip Siddiq, Kate Green, Tony Lloyd, Jack Dromey and Vicky Foxcroft, and Liberal Democrat Wera Hobhouse.
Some nursery schools have been forced to cut staff and services because of lost income and additional Covid-19 costs, coupled with a lack of certainty over the funding they will receive next year, unions say.
Almost half (46 per cent) of maintained nursery leaders said that by the end of March 2021, they were already in deficit for the year, according to a survey by Early Education, NAHT, NEU and Unison.
The average deficit reported was £76,000, and only 23% of respondents said they could continue to operate within their current funding levels, according to NAHT.
Ahead of the spending review later this month, school staff are calling for enough resources “to put in place a long-term viable funding solution”.
A Department for Education spokesman said: "We’ve made an unprecedented investment in childcare over the past decade, spending more than £3.5 billion in each of the past three years on our free childcare offers and increasing the hourly rate paid to councils above inflation for the past two years.
"We are also making millions more available through our early years recovery work to level up children’s outcomes.
"Maintained nursery schools provide valuable services to some of our most disadvantaged children and we remain committed to their long-term funding.
"We are providing local authorities with around £60 million in supplementary funding for their maintained nursery schools in the year to March 2022 – any decisions on future funding will be made as part of this year’s Spending Review."
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