Mayor of London Ken Livingstone and five council leaders have written to Transport Secretary Alistair Darling to ask that he make a decision before Parliament rises on 17 July.

The business case for the rail link from Heathrow to Stratford was handed to the government for scrutiny in February and a decision was expected in March.

But that date has since been put back to June and civic leaders are concerned the timetable for the whole project will be thrown off course.

If the government approves the costs and route options, public consultation would begin in time for a Hybrid Bill to be submitted to Parliament in April 2004.

'Essential' to Olympic bid

A leaked Treasury report suggested the multi-billion pound project was an essential part of London's bid to stage the 2012 Olympics.

And supporters say the multi-billion pound project is a vital part of the regeneration of east London.

Currently 30,000 people work in Canary Wharf, but that figure is expected to rise to between 100,000 and 120,000 within 15 years.

Tower Hamlets council leader Helal Uddin Abbas:"The current infrastructure does not have the capacity to bring those people in and out of the Docklands and that is just one area - there are a number of others beyond the Thames Gateway and lower Lea Valley."

"In order for construction to start on CrossRail, there are two Bills which need to be passed by the House of Commons and the timetable is very important.

"If we don't have an early commitment from the government it will be delayed, which we can ill afford."