A few arguments still to come over project's £16 billion costs

It is being hailed as the biggest engineering project since the Channel Tunnel.

Giant boring machines will drill under London in a feat of engineering not seen since the link between France and England was created.

But ministers may be shy about drawing any more comparisons between Crossrail and the Channel Tunnel given the latter's record on costs and completion.

The tunnel, first proposed in 1802 as part of a Napoleonic invasion plan, opened in 1994, £2 billion over budget and a year late.

Crucially, lingering doubts remain over the funding for Crossrail despite Gordon Brown's £16 billion deal with City chiefs last autumn. The Government had been demanding an extra £1 billion from businesses and the crucial breakthrough was the Corporation of London's agreement to find £350 million, £200 million from its own funds and £150 million from major City funds, to allow the project to progress.

The Department for Transport is now guaranteeing £5.6 billion. The Mayor, through Transport for London and the Greater London Authority, will raise up to £7.7 billion using a combination of borrowing on the back of the future revenues from a business levy and fares while Network Rail has pledged £2.3 billion.

But business chiefs in the City have already demanded reassurances about what will happen if the costs soar above £16 billion. The business tax plan, a supplementary-levy of 2p in the pound for 25 years on properties with a rateable value of more than £50,000, has also been attacked by councils and business.

If Crossrail follows a similar path to the Chunnel, then difficult financial wranglings lie ahead.

Even in 1964, announcing the the under-sea route, Transport Minister Ernest Marples said: "It remains to be decided when and how best the expense involved can be sustained."

Mr Brown has even suggested that only Ken Livingstone could deliver Crossrail.

Mr Johnson, though, insists a Tory government would not abandon the project, stressing: "I'm absolutely convinced that the Conservatives will want to fund London in the way that London deserves to be funded."