The mid-section of hull, known as “lower block 03”, is the first part of the £2.6bn HMS Queen Elizabeth aircraft carrier to go into the dry dock.
It took two years to build and is being moved by sea from a shipyard in Govan in Glasgow to Rosyth on the Forth, where the carrier is due to be pieced together on 21 August.
Minister for International Security Strategy, Gerald Howarth, said: “This marks an important milestone – the start of the assembly phase of the HMS Queen Elizabeth aircraft carrier.
“It is clear evidence that the UK shipbuilding industry has the expertise and experience to deliver a project of this size and complexity, delivering our next generation of Carrier Strike capability,” Mr Howarth continued.
Steven Carroll, project director at BAE Systems, said there was a real sense of pride at the yard.
He said: “Watching lower block 03 be towed down the Clyde gives us a chance to reflect on the huge achievements of the past two years since we cut the first steel on this first section.
To mark the completion of the construction, more than 50 cyclists left Govan on a 500-mile charity bike ride aiming to “beat the block” to its final destination.
The 65,000-tonne ship, the first of two new Queen Elizabeth-class carriers costing £5.2bn in total, is expected to be operational by 2020. Each one will provide the armed forces with a four-acre military operating base.
The 919ft (280m) carrier, along with its sister vessel HMS Prince of Wales, survived last autumn’s defence review despite massive cuts elsewhere in the Ministry of Defence budget. Both vessels will be converted to accommodate Joint Strike Fighter jets.