the Trades Union Congress (TUC) annual conference being held in the port city of Liverpool, northwest England.
"Unemployment could well exceed four million (in Britain) and it would take many years before there was any chance of returning to anything like full employment," said Barber, the TUC general secretary.
"That would scar for life a whole generation of young people."
Britain's unions face a tough future as recession threatens to cut public spending on key services such as schools and hospitals.
Furthermore, the governing centre-left Labour Party led by Prime Minister Gordon Brown is on course to be defeated by the main opposition centre-right Conservatives, opinion polls show, at an upcoming general election.
The nation's unions provide the bulk of funding for the Labour Party and fear for their future should the Conservatives win the election, which must be held by mid-2010.
Former Conservative prime minister Margaret Thatcher had famously crushed the unions' power during the 1980s.
"A double-dip recession would not just be deeper -- but also longer. Prolonged mass unemployment would not just do economic damage, but have terrible social effects," Barber told reporters on Sunday.
"I don't think that Britain is broken, but this would be one way to break it. Last time we suffered slash and burn economics we had riots in the streets here in Liverpool.
"I make no prediction that this would happen again, but I do know that prolonged mass unemployment will have terrible effects on social cohesion, family break-up and the nation's health," added Barber.
Rather than government cutting public spending, Barber said Britain's ballooning budget deficit caused by the recession could be tackled by scrapping controversial nuclear defence and identity card projects.
Additionally, money could be saved by further increasing taxes on the wealthy.
Britain has yet to follow France, Germany and Japan out of recession in the wake of the financial crisis, as the number of unemployed people in the country heads towards three million.
Britain's Business Secretary Peter Mandelson was on Monday to deliver a tough message on public spending in a speech to the think-tank Progress.
According to Britain's domestic Press Association news agency, Mandelson was to warn of "less spending in some programmes" and to admit that some government projects may have to be scrapped.
Prime Minister Brown was meanwhile set to tell union delegates in Liverpool on Tuesday that the government has "to make tough choices in public spending", according to extracts from his speech leaked to media.
On Friday, Brown hosted union leaders for private talks, described by The Times newspaper as "a charm offensive... to pacify Labour's disgruntled trade union paymasters, who are warning that the party may already have lost the next election".
Brown's office said the talks had been "constructive".
The TUC brings together more than 50 unions representing about six million mostly public-sector workers.