P & O European Ferries Wednesday attempted to break the three-month strike by ferry crews that has halted the company's services between the major U.K. cross-channel port of Dover and continental Europe.

The company flew several hundred replacement crews composed of non-union seamen to Rotterdam where five of the company's 11 ferries have been laid up since early February. Two of the ships are expected to be ready to sail today.However, P & O's confrontational action could cause the strike to spread. The ports of Hull and Folkestone were hit by unofficial stoppages Wednesday, while crewmen based at the United Kingdom's major containerport, Felixstowe, were discussing whether to support the Dover-based strikers.

Sealink Crews Take Action

Some seamen employed by Sealink British Ferries, the other major cross- channel operator, refused to cross picket lines. High Court action by Sealink to sequestrate assets of the National Union of Seamen on the grounds of secondary action was adjourned until today. Meanwhile, two French unions warned that P & O ships may be barred from French ports if the vessels are returned to service with non-union crews.

Should the company succeed in resuming ferry services using seamen who are not members of the National Union of Seamen, there could be repercussions throughout Britain's merchant fleet as 40 years of seafaring practices are overhauled.

Because industrial action has occurred at this time of year when vacation traffic is relatively light, the disruption to freight and passenger movements has been modest. Until this week, ferry services between Britain and continental Europe operated by other companies have been unaffected and delays short, except around the busy Easter period.

Dismissal Notices Sent

Tension mounted on Monday, though, after P & O sent out dismissal notices to more than 700 Dover-based seamen who refused to accept new working arrangements and advertised in the national press for job applications from both union and non-union officers and crew.

The dispute concerns P & O's plans to introduce reduced manning levels and longer duty hours in its bid to become fully competitive with the channel tunnel, which is due to open in 1993. The company estimates that its planned economies could cut labor costs by about $18 million a year.

Meanwhile, the Dover Harbour Board announced Wednesday that 100 jobs were to be eliminated at the port because of the loss of business resulting from the strike. The port has lost more than $4 million in berthing dues and traffic charges, and port management is considering suing the NUS for compensation.