The seals were two to three months old when orphaned

The RSPCA said four seals were tagged and released back into the Wash, off the Norfolk-Lincolnshire Coast, on Friday and another two on Tuesday.

Transmitters glued to their fur will record the seals' location and the duration and depth of their dives.

Researchers hope to study the impact of their upbringing in captivity.

The four males and two females had all been suffering from malnutrition, wounds and sores and had been abandoned by their mothers.

They're doing what we hoped they would - going off and foraging and being confident to do so in the wild; the sea is their oyster. Ros Varnes RSPCA spokeswoman

Staff at the East Winch Wildlife Centre in Norfolk monitored and fed them until they each reached 32 to 46kgs - weights thought to give them the reserves they need to adapt.

The tags glued onto the fur of Nemo, Hercules, Shrek, Snoopy, Skippy and Morocco, a male found in Dover last summer, should stay on until they moult in August.

The RSPCA has also teamed up with the Sea Mammal Research Unit at the University of St Andrews which, supported by the Department of Trade and Industry (DTI) is tracking wild common seals in the area at the same time.

Seals explore the area

Spokeswoman Ros Varnes said the seals had gone their separate ways since their release and were now exploring the Wash independently.

The most adventurous of the six so far has been Skippy, who has travelled more than 74 miles from her release site and was this morning tracked off the coast of Cromer in Norfolk.

The others have all moved between about 12 miles to 37 miles, swimming up to five times that distance on their zig-zagging routes, and were already diving to depths of up to 45 metres.

Laila Sadler, RSPCA marine scientific officer, said: "For the first time we will be able to compare the diving and ranging behaviour of completely wild animals with our rehabilitated seals.

Thousands of seals were recently killed by the PDV virus.

"This should give us valuable pointers as to how we can give other young seals in the future the best chance of survival."

Ms Varnes said: "We're very pleased with their progress.

"They're surprisingly mobile given that they've spent so much time in captivity. They're all moving around very well, exploring their new marine environment.

"They're doing what we hoped they would - going off and foraging and being confident to do so in the wild. The sea is their oyster."

Common seals in the area have in the past faced other challenges, including viral foes, which have affected their survival.