According to industry sources, the London Development Agency and electricity companies EDF and National Grid Transco chose Murphy ahead of opposition from its sole remaining rival for the deal, Morgan Sindall subsidiary Morgan Est last week.
One insider close to the bidding for the Olympic deal said: 'They both made final presentations late last week, and the LDA finally chose on Friday. It was a difficult decision for them as the two bids were very close on price.'
Costain and Amec were also contenders for the deal, but were both edged out of the competition at an earlier stage.
The project involves the boring of two 5.6 km long cable tunnels between electricity substations in Hackney and West Ham. The tunnels will vary between 2.5 and 6 m in diameter.
The deal is crucial because the main works on the main Olympic site cannot begin until the power cables are installed.
The LDA has already applied for planning permission to remove the pylons from the site. Work on the tunnels is due to start in November, and be completed in 2008.
The LDA is handling the procurement for the 2012 games with Transport for London until the Olympic Delivery Authority is established this autumn.
The LDA is due to announce its shortlist of bidders for the £100 million Olympic programme manager role 'within the next few weeks'.