The shipping to construction group is one of six consortia expected to apply for the contract along the 68-mile route between Cheriton, Kent and St Pancras in London.
Potential bidders have until 25 April to make a formal commitment to the project. The project is expected to be completed in seven years and provide a privately owned service running trains at up to 140 miles an hour.
P&O is already known to be backing a separate pounds 37m venture to build a rail station for the link at Stratford, east London.
But this is the first time that the group, chaired by Lord Sterling, has been associated with the main project.
The P&O consortium is understood to be led by its construction subsidiary, Bovis, although the make-up of the consortium has not yet been finalised.
Both P&O and Bovis remain tight-lipped about the plan, but a spokesman admitted: 'We are reviewing the situation'.
It has also emerged that SG Warburg, the merchant bank, and Bechtel, the big American construction group, have secretly joined a rival consortium fronted by Ove Arup, the consulting engineer.
Arup has never revealed the identity of its consortium members, despite being the first publicly to commit itself to the project. Ron Marsh, Arup's head of business development, said: 'This is a highly confidential matter, although I expect a further announcement will be made shortly.'
The third consortium planning to bid is Eurorail, whose members include General Electric Company and the construction groups Trafalgar House and BICC.
There is also growing speculation about strong overseas interest. A German consortium is being put together by Hochtief, the civil engineering group, and bids from Italy and France are expected.
The idea is to narrow down the contenders to a shortlist of four by June. They will then be invited to make formal tenders by the end of this year. The winner will be selected by mid-1995.
The four finalists will be asked to state the total cost of the project and how much cash funding they will require from the Government.
The winners will also receive assets worth pounds 800m - the bulk accounted for by St Pancras and Waterloo International stations, rolling stock and 25 acres of land near Kings Cross.