London — The first of four fleet tankers being built in South Korea to support Royal Navy operations still has not been accepted by the British Ministry of Defence, seven months after it was due to be handed over.

The Royal Fleet Auxiliary tanker Tidespring was expected to be accepted off contract by the British in January this year with the expectation it would enter service in September. But the vessel, part of the Royal Fleet Auxiliary's Military Afloat Reach and Sustainability (MARS) program, is still undergoing trials with builder Daewoo Shipbuilding and Marine Engineering (DSME) in South Korea.

It is unclear exactly what has triggered the delay in the handover of the tankers.

The MoD declined to answer questions about the delay in accepting the tanker or when the Tidespring would sail for the UK.

On her arrival in the UK the tanker is due to enter the A&P yard at Falmouth, western England, for fitting with sensitive equipment like self-defense weapons, ballistic protection and communications systems.

One source said the vessel had performed well on sea trials, particularly on handling and maneuverability tests.

"However, there remain a few first-of-class issues to resolve to ensure that all the naval and civilian approving authorities are content and to enable the MoD to accept the vessel off contract," the source said.

Despite the delays, a spokeswoman for the MoD said the department remains confident all the tankers will be in service by 2018, as planned, and that any additional cost resulting from the delays will not be met by British tax payers.

"We remain confident of delivering the MARS Tanker project within the original approved budget and expect all four tankers to be in service by 2018 as planned," the spokeswoman said. "As with all major projects, timelines are regularly reviewed and the nature of this contract means the MoD is protected from any unplanned cost increases."

Shipbuilder DSME was not immediately reachable for comment.

Britain ordered four of the 37,000-ton tankers from DSME in 2012 in a $597 million deal that sparked controversy here over the MoD's decision to subject the program to international competition rather than reserving the work for local yards.

The bidding proceedings saw no British contractors enter the final stage of the tanker competition, leaving the door open for the huge South Korean shipyard to outbid rivals for the work.

MoD policy is to have complex warships, like the Type 26 frigate, built locally, but put out to international tender vessels like the Royal Fleet Auxiliary support ships.

DSME did, however, team with British naval design house BMT Defence Services to use the Bath, western England, company's double-hulled AEGIR design for the tanker.

The South Koreans also are building a smaller version of the AEGIR for the Royal Norwegian Navy.

The new British tankers are equipped with a helicopter flight deck and hangar. Besides ship and aviation fuel, they are designed to hold ammunition and solid stores. The vessels are replacing old single-hulled tankers that no longer meet international environmental standards.

The ships are part of a wider and overdue modernization of the Royal Fleet Auxiliary needed to support the introduction of the Royal Navy's new 70,000-ton Queen Elizabeth aircraft carrier now nearing completion by a BAE Systems-led industry alliance.

The Royal Fleet Auxiliary delivers logistic and operational support to the Royal Navy around the world.

The precise schedule for delivery of the tankers was revealed earlier this year when the UK government published a series of letters giving details of recently appointed senior officers responsible for running some of Britain's major defense programs.

The letter to the new senior responsible officer for the tanker program showed the second of the vessels, RFA Tiderace, should have been handed over to the British in April and be in service by January next year.

Tidesurge, the third of the class, is targeted to be handed over in October and be in service by June 2017, with the final member of the fleet, RFA Tideforce, accepted off contract in April 2017 and in service by December that year.

Delays in the delivery of the initial tanker comes at a sensitive time for MoD shipbuilding procurement officials.

The department is in the process of moving forward with a program to build three logistics ships for the Royal Fleet Auxiliary, with an industry briefing July 28.

The requirement for the three ships, which will supply ammunition, food and other solid stores, was confirmed in the British government's  2015 strategic defence and security review.

The shipbuilding unions and others think the support ships should be earmarked for UK yards, but the cash-strapped defense ministry is more intent on ensuring low cost by going out to the international market.

"There will be an international competition to build the 'Fleet Solid Support' (FSS) supply ships, which UK companies will be able to enter, with a separate UK-only competition for customization work and trials," and MoD spokeswoman said. "This approach ensures the best value for money for the taxpayers."

Under current plans the MoD hopes to award a contract to build the ships by around March 2020.

The government is scheduled to roll out a national shipbuilding strategy by the end of the year, and it's unclear if the solid-support ships will play any part in that.

The main aim of the strategy is to sustain a long-term capability to build complex warships like the Type 26 anti-submarine warfare frigate and the smaller, and less expensive, Type 31 general-purpose frigate.

A spokesman for BAE Systems, Britain's only significant naval shipbuilder, said the company hadn't decided what, if any, involvement it would have in bidding for the solid-support ships.

"We are currently maturing our understanding of the MARS FSS programme and it is therefore too early to speculate on any further involvement at this stage," the spokesman said.