OSLO, Feb 12 (Reuters) - The Norwegian mainland economy contracted by 2.5% last year in what was probably the weakest development since 1945, Statistics Norway (SSB) said on Friday, although the decline was far less severe than in most other European nations. Norway’s fourth-quarter mainland GDP, a measure which excludes oil and gas production, expanded by 1.9% from the preceding three months, SSB said, outpacing a forecast of 1.2% growth in a Reuters poll of 13 economists. The government in March closed many public and private institutions in an effort to halt the COVID-19 pandemic, followed by a gradual lifting, although local lockdowns are still applied and the border remains closed to most travellers. “This is ... the biggest annual decline measured for the mainland economy since estimations began in 1970, and it is probably the greatest economic downturn since World War Two,” SSB economist Paal Sletten said of the full-year number. European Union outsider Norway fared comparatively better than the eurozone, where GDP declined by an estimated 6.8% in 2020 according to preliminary Eurostat data released last week. The crown currency strengthened slightly against the euro to trade at 10.2981 at 0755 GMT from 10.3043 just ahead of the 0700 GMT release. The nation of 5.4 million people had recorded 65,547 COVID-19 cases and 592 deaths so far in the pandemic, the Norwegian Institute of Public Health said on Thursday, making it one of Europe’s least affected countries. (Reporting by Terje Solsvik, editing by Gwladys Fouche)