Umbrella bodies for the charity and employment sectors have called on the Prime Minister to ensure there will be no drop in funding when the European Social Fund is replaced.
The National Council for Voluntary Organisations and the Employment-Related Services Association have written to Boris Johnson, urging him to promise that there will be no funding gap for organisations reliant on the EU-run ESF when Britain leaves the EU on 31 October.
The ESF, which focuses on improving employment and education opportunities, is due to provide €4.9bn worth of funding between 2014 and 2020. This amount is set to increase by 20 per cent from 2020 to 2027.
The 2017 Conservative manifesto promised the creation of an equivalent fund, the UK Shared Prosperity Fund, which was later confirmed by ministers.
But the letter said that since then there has been little concrete information about how much the fund will distribute, when it will be up and running and what its aims will be.
"Greater clarity and pace will be needed if there is to be a seamless transition from one funding regime to another to avoid the risk of services stopping for individuals who need them most," the letter said.
The letter asked Johnson to confirm that under his leadership the UKSPF would "help the UK achieve its post-Brexit potential by providing support for disadvantaged groups and communities".
It also called on him to ensure that employment and skills support allocated from the UKSPF would not fall below levels that would have been received if the UK had remained in the EU, and that there would be no gap between ESF funding and that from the UKSPF.
It warned that a funding gap could result in "permanent loss of expertise, infrastructure or partnerships for front-line organisations, and consequently, vital support for disadvantaged jobseekers and learners".
The letter said that leaving the EU would provide an opportunity to replace the ESF with "a world-leading initiative" that worked better for the UK, was more flexible and involved less bureaucracy.
"Getting this right will also build productivity and nurture a more inclusive economy, strengthening communities in the process and building a better Britain," the letter said, adding that investing in skills and jobs could help to heal division, improve health and wellbeing and build economic resilience ahead of the impact of leaving the EU.
The letter was signed by Sir Stuart Etherington, chief executive of the NCVO, and Elizabeth Taylor, interim chief executive of the ERSA.
Etherington said: "The government should seize this once-in-a-generation opportunity to develop a world-leading initiative that builds on the best aspects of the ESF while addressing some of the design flaws that have led to excessive and unnecessary bureaucracy."
Taylor said organisations funded by the ESF needed assurance that their services would continue to be funded after Brexit.
She said: "The promised UK Shared Prosperity Fund could establish itself as a world-leading initiative, but needs action now to be designed and ready to deliver in time so every part of society can share in our post-Brexit future."