Nasa's plans to return astronauts to the moon are quietly being revised and are in danger of slipping past 2020.

In meetings over the last few weeks at the Kennedy Space Centre, agency managers have told employees and contractors that they are delaying the first lunar launch of the Ares V rocket - a cargo hauler slated to be the most powerful rocket ever built - by two years.

Nasa's internal plans had called for Ares V to go to the moon in 2018, though the agency had announced a public goal of 2020. Internal deadlines are used by Nasa to keep programmes on track and to provide a margin of error for developmental problems.

But because of growing budget woes, the agency is resetting its internal date to 2020. And privately, engineers say that means the public 2020 date to send humans back to the moon is in deepening trouble.

The news is another major blow to KSC; the facility had hoped to get the moon-rocket programme up and running as quickly as possible to offset thousands of job losses from the space shuttle's retirement next year.

One contractor sent a BlackBerry message this week to the Orlando Sentinel following a meeting with KSC officials who told his group about the decision to delay Ares V.

"It was not received with enthusiasm," the contractor wrote. "[We] understand what that means for the work force."

In a speech last week, former Nasa administrator Mike Griffin blamed the White House - especially the Bush administration. He said that money available for Ares V and other moon projects had dropped from roughly $4bn (£2.7bn) through 2015 to just $500m.

"This was to be allocated to early work on the Ares V heavy-lifter, and the Altair lunar lander," he told the National Space Club at the annual Goddard Memorial Dinner. "With only a half-billion dollars now available, this work cannot be done."

Nasa officials would not comment.

"We cannot discuss any changes to Ares V until after the budget is officially released at the beginning of May," spokesman Grey Hautaluoma said. "Nothing definitive can be said about Ares V development cost at this point in time."