The operator told travellers on Sunday that “tourist travel” would not restart until Thursday.
But on Monday it posted on Twitter than it had resumed a three-services-per-day schedule for passengers and freight.
The DP World-owned company operated freight-only sailings on Sunday.
This was due to safety concerns after the company sacked nearly 800 seafarers and replaced them with cheaper agency workers.
The Maritime and Coastguard Agency (MCA) announced on Friday that the boat had been released following a reinspection.
Sailings by P&O Ferries remain suspended between Dover and Calais until at least Good Friday.
The resumption of services is dependent on regulatory approval.
P&O Ferries has accused the MCA of carrying out inspections with “an unprecedented level of rigour”.
The agency responded by insisting it works “in exactly the same robust way” for every ship.
The reduction in cross-Channel sailings has caused gridlock on major roads approaching the Port of Dover in recent days.
A 23-mile coastbound stretch of the M20 is closed from junction eight (Maidstone) to junction 11 (Westenhanger) to store thousands of lorries as part of Operation Brock.
This is causing chaos on surrounding local roads.
Dover and Deal MP Natalie Elphicke said: “The roads chaos affecting Dover and Kent caused by P&O Ferries is expected to continue into this Easter week.
“It is vital that Dover is kept clear amid the continuing roads disruption.
“Residents must be able to get about to work, attend medical appointments and visit family and friends.
“I am pressing for active policing of the key pinch points across our area and for blockages to be dealt with swiftly where they have arisen locally.
“P&O and DP World should be held to account for this disruption which they have needlessly and wrongfully caused.”
Roads in Kent have been hit by long delays in recent days due to a shortage of ferries caused by the suspension of sailings by P&O Ferries after it sacked nearly 800 seafarers without notice.