It is rare to find an old artefact made from an organic material such as wicker still intact, and the nine-strong team which discovered the basket on Monday evening delayed removing it until yesterday to prevent it from being exposed overnight.
The basket dates from the late 15th or early to mid-16th century, which is the period when fish from the moat were being cooked for the king's table, and was found west of the White Tower, in what used to be known as the West Moat. During the 16th century fish caught in that area were reserved for the king's household.
"To unearth something like this in a moat anywhere would be very interesting, but to find it in a world heritage site is extremely important," said Graham Keevill, director of the excavation. "The discovery backs up all the documentary sources we have on this period."
According to Mr Keevill, artefacts made from organic materials such as wicker rarely survive due to bacterial decay from the soil, but conservation experts believe the basket was preserved because of water levels in the clay and by being four metres beneath the surface of the soil.
After they noticed pieces of wickerwork protruding from the trench on Monday, the archaeologists began the preparations to move the basket. To protect it, they will remove the basket in the large block of soil it was found in by slipping a timber board underneath.
They will then probably use an airtight container - described by one expert as a "giant lunchbox"- to ensure the basket stays waterlogged and to prevent oxygen from causing decay. The container and basket will then be moved to Hampton Court in south-west London, where more protracted conservation treatment can begin.
The West Moat, the site of the find, is proving to be a treasure trove for archaeologists. "This is one of the best places at the Tower of London to dig because it gives you a cross section of the entire area," said Mr Keevill, who believes it is a prime archaeological site. His team has been on site for just under a year and has dug about 60 trenches around the Tower of London, but the site of the West Moat is the biggest single trench dug so far.
The fishing basket counts among the top five finds unearthed on the site. Last year, members of the Oxford Archaeological Unit unearthed a tower dating from the 13th century during the reign of Henry III, and only last week a jug and a money box, estimated to be at least 500 years- old, were discovered in the same area.