Rail travellers face queueing for trains in the future unless the HS2 link is built between London and cities including Birmingham, the head of the project has warned.
Sir David Higgins said the new line was vital to alleviate overcrowding on routes in and out of the capital.
Without it, demand on Britain's straining railway network will leave people waiting on platforms and unable to board full trains, he told a national newspaper.
Sir David, who took up his role as chairman of the HS2 project at the start of the year, also called for both northern and southern sections to be built at the same time, which would cut costs.
In a report next month the newspaper said he was likely to insist that work on both sections must begin together.
If the links to Manchester and Leeds are not built at the same time as that from London to Birmingham, there will be a danger that people in the North would see the scheme as a "bypass to Birmingham".
He told the Telegraph that there was "no reason" why the £42 billion line should be seen "purely as a London to Birmingham project".
Sir David said that political differences were the "biggest risk" to the controversial project, and that any delays caused by arguing in Parliament would lead to increased costs for the taxpayer.
Speaking about current levels of overcrowding, Sir David said that the Birmingham to London line he regularly travels is "unbelievably packed, absolutely heaving".
He said the west coast line is performing at capacity and is showing the strain as the busiest mixed use railway line in Europe, whereas HS2 offers 18 trains an hour in each direction.
He feared the problem of overcrowding could get worse, saying: "You won't get on trains. It will be like the Piccadilly Line at peak hour. Usually I stand for three trains before I can get on. You'll be doing that at Milton Keynes. You'll be forming queues to get on trains."
HS2, whose £50 billion cost includes £7.5 billion for the high-speed trains, has a first phase which runs from London to Birmingham, which is due for completion in 2026.
A second, Y-shaped line north of Birmingham to north west and north east England is due to be finished around 2032/33.