The big conversion job has also seen contractors Careys pick up a £1.5m contract for preparatory work for Balfour Beatty and Imtech G&H take a £25m contract for electrical and plumbing work.

The roof contract will eventually be transferred over to the successful bidder of the main conversion contract which is being procured under a separate contract.

Five firms are in the running for this £100m job, including Balfour, Buckingham, ISG, Shepherd and Sir Robert McAlpine.

McAlpine is widely expected to take the main contract as former Olympic Stadium builder, although the roof award makes Balfour a serious contender.

Balfour will begin to dismantle the existing roof and floodlight towers in the autumn, and is expected to take around four months to complete.

Then an extensive cable net system will be fitted to support the new roof which will be constructed on the floor of stadium and then lifted into position.

Several 600-tonne cranes operating within the stadium will then tandem lift the supporting roof steelwork into place.

The roof will be twice the size of the original at around 45,000 sq m covering every seat in the ground, dramatically improving acoustics for matches and concerts.

Retractable seating arrangements in the stadium will allow for seats to be moved backwards and forwards depending on the type of event taking place. It will allow pitch side football viewing while retaining the iconic international class running track.

All roof works will be complete by spring 2015 in time for the 2015 Rugby World Cup.

Transformation works at the stadium are due to be complete by spring 2016 for the start of the new season in 2016/17 when it will become the home of West Ham United Football Club.

Balfour Beatty Chief Executive, Andrew McNaughton, said: “This award enables us to continue to develop our role in the London 2012 Olympic legacy.  Our utilisation of leading Building Information Modelling techniques has allowed us to develop a bespoke construction methodology that will reduce the programme risk to this technically challenging scheme.