Unions have accused the government of doing "absolutely nothing" to strengthen workers' protections a year after the sackings of nearly 800 P&O Ferries workers without notice.
The Trades Union Congress (TUC) said promised sanctions following the controversial dismissals failed to materialise.
It warned there were "gaping holes" in the new Seafarers Bill and ministers had failed to close loopholes in minimum wage law, warning without government action, another P&O Ferries-style "scandal" could happen.
The union federation set out four breaches of law by P&O Ferries it claimed had gone unpunished - a duty to consult over redundancies, unfair dismissal of workers, failure to notify the relevant government authorities, and breach of director duties.
The Rail, Maritime and Transport (RMT) union is staging rallies in Dover and Hull to mark the anniversary of the "illegal and deplorable" mass sackings.
P&O's decision to fire 786 crew members without notice or consultation over Zoom last March and replace them with much cheaper agency workers sparked widespread outrage.
There was fresh condemnation this week as the Dubai-based owner of the ferry operator, DP World, said its annual profit increased by 37% in 2022.
It reported $1.23bn in net profit for the year ending 31 December - up from $896m in 2021. Revenue grew by 58.9% to $17.1bn.
'Free pass for rogue employers'
The TUC said urgent action was needed to clamp down on "fire and rehire" style practices.
Its general secretary Paul Nowak said the government liked to "talk the talk" on protecting workers but has done "absolutely nothing" since the P&O sackings.
He said: "The mass sacking by P&O Ferries was a national scandal. It should have marked a new chapter for employment rights in the UK, but this Conservative government has failed working people and given rogue employers a free pass to act with impunity.
"Despite behaving like corporate gangsters, P&O Ferries has been allowed to get away scot-free because of our lax labour laws.
"Instead of boosting worker protections and closing legal loopholes, ministers sat on their hands and did next to nothing, and to add insult to injury, ministers are now actively borrowing from the P&O Ferries playbook.
"They are brazenly attacking the right to strike and threatening to rip up hard-won workers' rights like holiday pay, equal pay for women and men and rest breaks.
"Without stronger regulation, another P&O Ferries style scandal is on the cards."
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A P&O Ferries spokesperson said the company had invested £250m in its fleet, adding: "Significant changes in the last year have saved this business, including the 2,200 jobs we secured in coastal communities across the UK. As a result, we are now serving the needs of our passenger and freight customers much better than ever before."
A government spokesperson said: "We reacted swiftly and decisively against P&O Ferries' appalling treatment of its staff, and have made substantial progress on the nine-point plan we set out last year to improve seafarers' pay and conditions.
"Having brought forward legislation to ensure seafarers are paid at least an equivalent to the UK national minimum wage, and establishing a new statutory code to deter 'fire and rehire', we are now working with our near European neighbours to further protect their welfare and pay."