Costa made the promise in a speech opening the fortnightly debate in parliament in which he reiterated his intention, announced on Friday, of revising the tax framework for companies that set up shop in the interior, with a view to reducing their liability for corporate tax.

"We shall review the applicable fiscal framework, reinforcing positive discrimination for the interior," the prime minister told deputies.

"Among other measures, we intend that companies in areas with the lowest population density can benefit from substantial reductions in IRC" - corporate income tax.

The effective rate could even be "zero", he said, depending on the number of jobs involved.

Costa also pledged the government to working for "the attraction of investment that creates jobs" in the interior, with a view to ensuring people stay in these areas.

"As part of the reprogramming of Portugal 20/20, we propose a programme of incentives for business investment exclusively aimed at areas in the interior, to support investments [totalling] 1,700 million euros in the period from 2019 to 2021," he said. "And because support to investment cannot be based only on activities to do with innovation and exports, we shall launch a credit line in the amount of 100 million euros aimed exclusively to small and micro-enterprises situated in the interior for the financing of projects for the creation, expansion or modernisation of production units."

In his speech Costa hailed proposals recently presented by the self-declared 'Movement for the Interior', which has called for a new programme of medium- and long-term state investment in parts of the country with lower population densities.

The prime minister acknowledged that reforming the country's forests is "fundamental" to boosting the economy in the interior. Such reforms, he recalled, were launched in October of 2016, including the registration of ownership of forest land, and the approval of regional forest plans.

There is, he argued, a consensus in the country as to the importance of combatting imbalances between the more densely populated coastal areas and the interior.

"The moment has arrived for us to create the conditions for territorial cohesion to go from being a simple idea to reality," he declared. "We know well that the movement of abandonment of the interior has been going on for decades and that a structural phenomenon of this scale cannot be reverted in a few years. But we also know that there has never been so much effort as know - from the President of the Republic, from parliament, from the government, from local authorities and civil society - to acknowledge territorial cohesion as a veritable national goal."

Last year was the most deadly year on record in terms of forest fires, with more than 100 people killed and over 500,000 hectares of forest burned, most of it in the hilly interior.