The employment minister has claimed a small number of activists are trying to "destabilise" firms involved in a controversial work experience scheme.
The scheme allows unemployed youngsters to do unpaid work for up to two months without losing benefits but has been criticised by some as "slave labour".
Chris Grayling said firms were "jumpy" because of a "false campaign" he blamed on the Socialist Workers' Party.
But Right to Work protesters said it was a "broad-based" campaign.
The scheme, aimed at 16- to 24-year-olds on jobseeker's allowance (JSA), allows them to do unpaid work experience with a company for up to eight weeks - without losing their benefit and potentially with some expenses paid.
But if jobseekers choose to take part and then fail to turn up without good reason after the first week, their benefits could be docked for a period. This has led critics to question whether the placements are really "voluntary".
Of the 34,200 people who took part in the scheme between its launch in January 2011 and November that year - the government says 200 had their benefits docked.
Various firms have expressed concerns about government-backed work placement schemes in the past week, amid claims that they exploit people on benefits.
On Thursday Ken McMeikan, chief executive of the bakery chain Greggs, told BBC Two's Newsnight programme he and other business leaders would meet the government next week to discuss the scheme.
He said: "If after a week or more you decide as an individual that it's not working for you and you leave the scheme, we don't believe at Greggs that the benefits should be taken away."
But he added it was a "small minority of people" of people criticising the scheme and most youngsters who had been through it "like it and they want us to continue offering it".
Mr Grayling told BBC Radio 4's Today programme that joining the scheme was "entirely voluntary" and many of those who took part went on to get jobs with the firms.
"All of the evidence we can see is that this does better than simply leaving people on JSA, it actually helps more young people get into work."
He said "jumpy" companies were coming under pressure from an internet campaign which he claimed was being run by the Socialist Workers' Party.
"The High Street retail sector is going through a tough time at the moment, if you're running a company and you're getting streams of emails attacking you, it's very unsettling. It's a false campaign.
"I don't accept that the scale of the campaign is very large, it's a small number of activists who are deliberately targeting these companies and trying to destabilise them."
Jobseekers who dropped out of the scheme "without good reason" would be investigated - and in some cases face the same sanction they would for not turning up to sign on at the job centre, he said.
And he argued that the only compulsory scheme they operated was a short-term scheme, "mandatory work activity", used when Jobcentre Plus advisers felt someone's job search has "gone off the rails" - which was work carried out on "community-benefiting projects".
Waterstones, Maplin, Matalan, Tesco and Argos are among firms to have expressed concerns about government-backed work placement schemes in recent days.
On Friday Poundland said it was withdrawing from participation in another scheme - the government's work programme - which is aimed at people who have been unemployed for more than a year. Claimants who refuse to take part in recommended work experience can face benefit sanctions.
Sainsbury's, meanwhile, said the small number of stores that took part in the work experience scheme following local approaches had since ceased participation, as it was not company policy.
Michael Bradley of the Right to Work campaign said he was "proud to be a socialist" but it was a "broad based" campaign backed by six trade unions and chaired by Labour MP John McDonnell.
He told BBC Radio 4's World at One: "Like lots of socialists, people think what the government is doing with the workfare scheme is pretty outrageous - that's why people are going onto the streets and protesting about it."
And Socialist Workers' Party national secretary Charlie Kimber said: "Grayling should know that the campaign against forcing the unemployed to work for nothing is supported by very large numbers of people, not just the SWP."
Kirsty McHugh, of the Employment Related Services Association, which represents "welfare to work" companies, said if benefit rules were stopping companies wanting to provide placements the government should look at them again.