The �600m European space mission to chase a comet will be delayed by up to two-and-a-half years, it emerged on Wednesday.

The time lag gives scientists the opportunity to find a new comet for the Rosetta spacecraft to orbit and land on.

The decision to postpone the flagship mission was taken because of doubts about the reliability of the spacecraft's launch rocket, European Space Agency (Esa) officials have revealed.

There will be no Ariane 5 launches until the review is complete

But they say there is no question of the mission not going ahead at some point in the future.

"There isn't the slightest doubt in my mind we're going to get this thing off the planet," Esa's director of science, Professor David Southwood, told journalists in Paris. "Not to be able to launch at all was never on the cards."

The delay is expected to add as much as �60m (100m euros) to the overall budget cost, a bill that must be met by Esa member states.

Meanwhile the search begins in earnest for a new comet to study. Dr Gerhard Schwehm, the lead scientist on Rosetta, said there were at least four or five good candidates.

Cosmic billiards

They are among the family of comets that orbit the Sun and Jupiter, which are relatively easy to reach from Earth.

"What we want is an active comet, that means a comet that produces a lot of dust and a lot of gas," he said. "When we have identified a target, there are different ways to get there - we play a cosmic billiard game."

Rosetta is designed to drop a robot on a comet (Photo: Astrium)

The new targets will be reviewed in mid February by Esa's science policy committee.

Members will also have to decide how to meet the extra costs of the re-designed mission and whether other science programmes will have to be scrapped.

The spacecraft - waiting for launch at the Kourou space port in French Guiana - will now be de-fuelled and taken apart.

Its payload of 11 scientific instruments and a lander will be stored for future use.

Bold plan

The plan to chase down Comet Wirtanen and put a lander on its icy surface in 2012 was a bold one that generated huge excitement.

There was perhaps a level of doubt about the manner in which overall the system was put together Prof David Southwood

But Esa had little choice but to suspend Rosetta preparations when concerns surfaced about the reliability of its launch rocket.

The first delay followed a preliminary inquiry into the failure of Europe's new super-rocket, the heavy-lift Ariane 5-ECA (ESC-A), on its maiden flight in December.

An engine fault was identified as the main cause of the disaster but doubts later emerged about other aspects of the rocket.

"There was perhaps a level of doubt about the manner in which overall the system was put together which left concerns in my mind and I don't think were appropriate when you have a billion euros riding on the rocket," Professor Southwood said on Wednesday.

Full review

The European rocket operator, Arianespace, will now carry out a full review of its Ariane 5 launch system.

It will probably mean that another Esa mission - to map the Moon - will also be delayed.

When it finally does fly, Rosetta should add substantially to our understanding of comets.

Studying the materials trapped in these icy bodies could reveal details about the origins of the Solar System and how water - and possibly even life - arrived on Earth.