Government plans to make striking workers wear official armbands on the picket line are similar to Nazi attempts to victimise trade unions, the head of Unite has said.
Len McCluskey told Labour's conference his members would not comply with the proposal in the Trade Union Bill.
He said the Nazis made trade unionists wear armbands with red triangles "in the concentration camps of Dachau".
Ministers say picket leaders already wear armbands to identify themselves.
The bill would also impose a minimum 50% turnout in ballots - with public sector strikes also requiring the backing of at least 40% of those eligible to vote.
Under current rules, strikes can be called if the majority of those taking part in a ballot vote in favour.
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The bill, which would apply to unions in England, Wales and Scotland, also proposes the introduction of fines of up to £20,000 on unions if they repeatedly break the rules, including by failing to ensure picket supervisors wear an official armband.
Mr McCluskey, whose union is Britain's largest, told Labour delegates in Brighton the bill was "an unnecessary, illiberal and spiteful attack on free trade unionism".
He said: "I will be on the picket line when Unite members are on strike and I will not be wearing an armband with a red triangle like the trade union prisoners.
"Conference, remember, that's what the Nazis did - trade unionists in the concentration camps of Dachau - made to wear armbands with red triangles.
"We won't be doing that."
Mr McCluskey did offer to co-operate with ministers on ballot thresholds in return for modernisation of the voting system.
"If your concern about turnout in strike ballots is genuine, then scrap the archaic and undemocratic reliance on postal ballots and give trade unionists the right to secure, secret, workplace balloting.
"Modernise the voting system and there will no longer be a turnout problem. If ministers agree then we can reach an agreement on thresholds."
The Trade Union Bill passed its first Commons hurdle earlier this month - by a majority of 33 votes at second reading - despite fierce Labour criticism.
Business Secretary Sajid Javid insisted the bill was "not a declaration of war" against unions, but necessary to stop "endless" threats of industrial action.
The bill would also:
- Double the amount of notice unions have to give before a strike can be held - from seven to 14 days
- Allow employers to use agency workers to replace striking staff
- End the so-called check-off system for collecting union subs direct from a salary
Unions already work to a Code of Practice - which recommends armbands or badges to identify authorised pickets - but the new bill would give a regulator the power to impose sanctions for non-compliance.
A Department for Business spokesman said: "None of these changes are about banning strikes, but we need to get the balance right between the interests of unions and the interests of the majority of people who rely on important public services.
"These modernising reforms will ensure strikes only happen as a result of a clear, positive decision by those entitled to vote."