The Prime Minister is being urged to provide "urgent clarity" on whether Cornwall will continue to receive local growth funding.
Cornwall Council leader Leigh Frost (Liberal Democrats) has written to Sir Keir Starmer asking whether the county is set to be phased out of the financial support.
The letter comes in response to recent announcements suggesting future growth investment may be targeted only at mayoral city regions.
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Mr Frost wrote: “If government does intend to remove this support from Cornwall, the consequences would be severe. It would represent a missed opportunity for this government to choose to work together with Cornwall to continue the huge strides made through the local growth funds over the past two decades in transforming our economy."
Mr Frost warned that moving to a two-tier funding model could "hold back productivity and growth" by weakening regions such as Cornwall that still need investment.
“Our region still faces some of the lowest wages, highest levels of in-work poverty, and deepest structural economic challenges in the country," the letter stated.
"Progress over the past two decades has been achieved thanks to consistent, predictable, place-based investment. Removing that investment would risk reversing hard-won progress, damaging confidence and deterring private investment.”
Councillor Tim Dwelly (Independent) supported the call for answers from government.
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He said: “We want to know if ministers really are planning to phase Cornwall out of future growth funding. That would be an extraordinary decision, ignoring the fact that Cornwall has consistently delivered value for public investment and has huge potential to drive growth, innovations and skills not just locally, but nationally.
"Like Councillor Frost, I am asking government to confirm its intentions – Cornwall deserves a fair chance, not to be quietly pushed aside.”
Councillor Frost said Cornwall deserved "a fair, reliable settlement on an equivalent basis to mayoral city regions".
He added: “We will not sit quietly while our communities and industries are treated as an afterthought.”
In March, North Cornwall MP Ben Maguire (Liberal Democrats) wrote to chancellor Rachel Reeves demanding why the Duchy was being "overlooked" after it missed out on Government transport funding.