Householders who employ black-market nannies could face prison under proposals published today designed to clamp down on Britain's invisible army of migrant workers.

Tony McNulty, the Immigration Minister, said the powers were directed towards catching companies, gangmasters and employment agencies that hire illegal migrants on a large scale, but accepted that they could also be used against individuals.

A key factor in deciding whether prosecution is appropriate under the Immigration, Asylum and Nationality Bill would be whether the employer was responsible for paying the National Insurance contributions of a worker found to be in Britain illegally, Mr McNulty said.

"If it is a full-time childminder living in the home and National Insurance contributions are paid, then that is an employment contract, clearly," he said.

Under the terms of the Bill, employers - of whatever scale - would be forced to pay an on-the-spot civil fine of up to £2,000 for every black market employee found on their premises.

There would also be a new offence of knowingly employing an illegal worker, carrying up to two years’ imprisonment or an unlimited fine. This updates the legislation from the 1996 Asylum and Immigration Act, which set the maximum fine at £5,000.

Mr McNulty backed the proposals with the announcement of a new 12-strong taskforce of investigators dedicated to tracking the black market labour force.

Estimates of the number of illegal migrants working in Britain are vague, varying from the low thousands to hundreds of thousands.

Last year, there were 1,600 operations to combat illegal working which uncovered 3,330 employees. Three-quarters worked in hospitality, car washes or garages, the sex industry, shops, as cleaners or in food production.

Mr McNulty said that it was less likely that householders would be fined for hiring illegal immigrants working as plumbers, builders or other tradesmen because law defines this as a purchase, not a contract of employment.

Karen Bransgrove, director of the UK and Overseas Nanny Agency, based in Central London, told Times Online: "If anyone employs a nanny without checking that they are working legally they are quite frankly mad.

"Everyone's passport will state clearly whether they are entitled to work here or not. If they are not, then parents are taking a risk with their children. If the care is not done legally and above board and something goes wrong then there could be terrible implications."

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