A second P&O Ferries ship failed a safety inspection and has been detained after failing safety checks, according to the Maritime and Coastguard Agency (MCA).
The Pride of Kent is one of eight ships to need inspections before re-entering service, after 800 staff were sacked with no notice.
The firm replaced staff with agency workers paid less than the minimum wage, but the government says it has prepared measures to block its plans.
The MCA was inspecting the Pride of Kent to make sure it was safe to go to sea without passengers or cargo.
Transport secretary Grant Shapps confirmed that the vessel had been detained by the MCA.
He added in a tweet: “Safety will not be compromised and further checks will continue.”
Another P&O Ferries vessel, the European Causeway, failed an MCA Port State Control inspection last week.
The ferry was detained in Larne over “failures on crew familiarisation, vessel documentation and crew training”.
RMT union general secretary Mick Lynch said: “It’s rare enough for the MCA to impound a ferry but P&O have now had two in a week after the jobs carve up which speaks volumes about the dire state of their operation.
“It’s now high time for these important vessels to be taken over under public control with the sacked crews reinstated as the only way to get these crucial ferry routes back running safely. ”
Shapps has pledged to bring a “comprehensive package of measures” to Parliament “to block the outcome that P&O Ferries has pursued”.
He wrote to the ferry company’s chief executive, Peter Hebblethwaite, on Monday to make clear that “this government will not stand by while the requirement to treat seafarers with due respect and fairness is brazenly ignored”.
Last week, Hebblethwaite told a committee of MPs that the company knew it was breaking the law by not consulting before the sackings.
P&O Ferries said on Monday afternoon: “We fully welcome the government’s commitment to increasing the minimum wage for all seafarers working in British waters.
“From the outset we have called for a level playing field when it comes to pay and conditions on British ferry routes.
“Our announcement is not about reducing seafarer’s wages, it is to enable us to have a fully flexible crewing model that allows us to meet the demands of our customers.
“The predicted savings we announced are not solely coming from the reduction in wages, but from removing job duplication and the benefits we will see from increased flexibility.”