An ageing nuclear cargo ship has been rescued from the scrapyard to save money transporting plutonium and other radioactive materials around the world, prompting accusations that maritime safety is being jeopardised.

The 25-year-old vessel, Pacific Pintail, has been brought back into service to make dozens of international shipments between nuclear plants over the next three years. Last month it moved three kilograms of plutonium under armed guard from Sweden to the US.

The Pintail was laid up at Barrow in Cumbria two years ago for decommissioning, but has been resurrected after a £44m plan to build a new nuclear cargo ship was abandoned as too expensive. An internal report seen by the Guardian admits that the continued use of the old ship "will present some PR difficulties".

But the problems could be managed, the report said, by preparing a "stakeholder communications plan" and by giving the boat a new name. It has subsequently been renamed the Oceanic Pintail.

The report was drawn up in November 2010 by International Nuclear Services, a company owned by the government's Nuclear Decommissioning Authority. It says that British nuclear cargo vessels "have traditionally been retired from service before their 25th birthdays"..

But it proposed extending the life of the Pintail, which turns 25 this year, until at least 2015. This is an "interim solution" and "a low-cost opportunity", it said.

The ship will be used to take multiple cargoes of radioactive waste from the Sellafield nuclear plant in Cumbria to countries in Europe. There are plans for a series of other international shipments, though details have been kept secret.

The INS report also discloses doubts over the continued use of another nuclear cargo ship, Atlantic Osprey, which is 26 years old. Keeping the ship in service "presents difficulties", it says, because the French nuclear safety authority, ASN, has "reservations" and because of "reduced public acceptance/political credibility".

But the Osprey is still currently scheduled to ship mixed plutonium and uranium oxide (Mox) fuel from Sellafield to Germany for use in the Grohnde nuclear power plant in Lower Saxony. The shipment has been postponed from last year because of Mox manufacturing delays.

INS has been accused by anti-nuclear campaigners of keeping both boats in service beyond their sell-by dates. Cumbrians Opposed to a Radioactive Environment are demanding that the Pintail be scrapped and the Osprey shipment be cancelled.

The continued use of the Pintail is a "stopgap" and "cheapskate option", according to Core's Martin Forwood.

"This seriously compromises the safety of nuclear shipments and poses significant risks to the marine environment and communities along transport routes," he said.

INS is playing "fast and loose" with maritime safety by putting business needs first, he added. The internal INS report was released to Core in response to a request under freedom of information law.

INS confirmed that the Pintail has been brought back into service, and that the Osprey is due to make a shipment to Germany. But the company insisted that it is "categorically untrue" to say that safety had been compromised.

The Pintail has a "flawless safety record", said Ben Todd, an INS spokesman. It is quite capable of operating safely for longer than 25 years and its operations are independently regulated by the Maritime and Coastguard Agency.

The ship has been renamed because its owners and area of operation have changed, Todd added. The Osprey is also "fully approved and certified to make Mox shipments".