Experts believe families are marrying off disabled relatives to ensure they have long-term carers or, in some cases, to make money from immigration scams.
The situation, as reported by the BBC Radio programme Face the Facts, has led 50 councils to adopt guidelines designed to tackle the problem.
The Government’s Forced Marriage Unit (FMU) said it had received a “growing number of referrals” involving adults with learning difficulties.
Mark Goldring, chief executive of charity Mencap, said the issue was vastly underestimated.
He said: “We know that there are many more (cases) where somebody is at real risk.
“Because the marriage often takes place outside this country, it’s not until it’s done that many of the wider community will actually get to know about it.
“Public awareness and the officials’ awareness is massively under-reported.”