Brussels had warned that the tender must be approved this month or the more than 5.5 billion euro ($6 billion) project would not receive 729 million euros from the Connecting Europe Facility.
The government predicts that the line, expected to be fully operational after 2030, will reduce the 300 km (186 miles) journey between the capital and Portugal's second-largest city to only 75 minutes, down from more than three hours.
The railway will be built under a public-private partnership scheme in which private companies will have up to five years to build it and will have a concession to operate it for 25 years.
The first phase, consisting of two sections covering more than 120 km at an estimated cost of 3.55 billion euros, is expected to be completed in 2028. The government said it approved the tender for the first 70 km section between Porto and Aveiro.
The second phase - around 120 km between Coimbra and Carregado - is expected to cost 1.9 billion euros, but costs of the last phase to Lisbon are not yet known.
Portugal sees train travel as a way to reduce motorway and air traffic, helping it to become carbon neutral by 2045.
Environment agency APA last year approved the stretch between Porto and Coimbra, but the project is facing growing opposition from several municipalities and environmentalists.
($1 = 0.9126 euros)
(Reporting by Sergio Goncalves; Editing by Andrei Khalip and David Goodman)
By Sergio Goncalves
