Robert Jenrick will make a speech on Reform's welfare plans on Monday
Foreign nationals would be excluded from most benefits under a Reform UK government, the party has said.
The ban, part of a proposed overhaul of Britain's benefits system, would include European Union (EU) nationals with settled status in the UK.
It would require renegotiating the UK's Brexit deal with the EU, potentially meaning that British expatriates living in the EU could lose equivalent rights.
Reform claims the policy would save £21bn a year by its fifth year, but Labour insists the "vast majority" of migrants have no access to benefits and the Conservatives have dismissed the plans as "cobbled together".
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It is the latest announcement ahead of a speech on Monday by Reform's shadow chancellor Robert Jenrick to formally unveil the party's welfare plans.
"Forcing British workers to pay for the benefits of foreigners is not just economically illiterate but plain immoral," Jenrick said in a statement.
"People are more than happy to support their neighbours in hard times, but the British taxpayer cannot afford to subsidise everyone on the planet, especially those who have not paid in."
This year the government expects, external to spend a total of £322bn on welfare in Great Britain, or 10.6% of GDP, just over half of which goes to pensioners.
Reform says it has spent six months drawing up a 50-page plan that it claims will save a total of £50bn a year.
On Saturday, the party said it would replace the disability payments system, Pip, if it entered Number 10 at the next election.
The latest proposals would ban foreign nationals from almost all welfare payments including housing benefit, pension credit, jobseeker's allowance, child benefit, free childcare and disability benefits.
Only a small number of exemptions would apply, including the war widows pension and Armed Forces compensation.
