Up to 100,000 people who employ personal carers are being warned that they may now be legally liable to pay them pension contributions.
From 1 June, small businesses employing up to 30 staff will gradually be obliged to offer their staff pensions, under the auto-enrolment programme.
That includes people who only employ one person, such as a carer or a nanny.
The Pensions Regulator, which oversees the programme, says those who do not comply could receive a fine of £400.
However, it says it is working with 200 support organisations to make sure people get proper advice.
Under auto enrolment, all employees over the age of 22 who earn more than £10,000 a year will now have the right to a workplace pension - unless they decide to opt out.
So far, 5.2 million workers in 45,000 larger companies have joined the scheme. Between now and 2018, an estimated 3.8 million more workers will be enrolled by about 1.3 million employers.
Anyone who took on an employee for the first time after 1 April 2012 will not have to pay pension contributions for another two years.
And many elderly or disabled people - whose carer is organised through the local authority - may find the pension contributions being paid for them.
But it is far from clear whether all local authorities will be prepared to take on the extra costs of a pension.
All the Department for Work and Pensions will say is that councils "should consider" the costs.
In the meantime, warning letters sent out by the Pensions Regulator have come as a surprise to many people who employ carers.
"It's come out of the blue, and they've just been unaware of it," said Edward Graham, head of advice and information at the charity, Carers UK.
"It's caused a very high degree of anxiety amongst many people who've received these letters."
Those who employ a carer on a salary of £15,000 a year will have to make pension contributions of £185 a year within two years of the scheme starting.
The first £5,824 of income is ignored, as is any income over £42,385.
Pension costs for nannies - who can earn much more than carers - might be considerably higher.
Louise Taylor, a director at agency Kensington Nannies, puts the cost at between £300 and £500 a year, but believes few people will be put off.
"In the grand scheme of things, another £400 is not going to stop you from hiring a nanny," she told the BBC.
However, she said that many people finding nannies on the internet will be unaware that they are liable for pension contributions.
Equally, many nannies will be unaware of their rights.
"A nanny new to Britain will not know she has a right to a pension," she said.
People who employ cleaners or gardeners will not be affected, unless they pay them more than £10,000 a year.