By Ana Vicario

MADRID (Reuters) - The number of Spanish nationals moving abroad rose last year to its highest level since comparable figures began in 2008, the National Statistics Institute said on Thursday, with many traveling to Britain, where they now face an uncertain future following last week's vote for Britain to leave the EU.

Almost 100,000 of the country's citizens left in 2015 with one in eight moving to Britain, 10 percent to France, 9.6 percent to Germany and 9.3 percent to the United States.

Britons last week voted to leave the European Union after "Brexit" campaigners said freedom of movement laws in the bloc had allowed an unsustainable number of EU citizens to move to Britain, putting a strain on social services.

All told, Spain's population declined by 11,142 in 2015 to 46.438 million, a drop of 0.02 percent, and the fourth consecutive year that the number has fallen.

Though it was the shallowest drop in the series, the decrease was part of a trend that has ground on since the financial crisis pitched the country into a five-year slump.

The downturn did not bottom out until mid-2013. By then, construction activity had crumbled and unemployment was soaring, sparking an exodus of thousands from the Iberian peninsula.

In the four years up to Jan. 1, 2016 the population shrank by about 380,000, and last year the net outflow of Spanish citizens was almost 47,000. Some 52,000 also returned in 2015.

Some of the Spanish nationals leaving the country were born in Latin America and elsewhere before acquiring passports.

The net outflow of Spanish nationals in 2015 was largely offset by the first net influx of foreign nationals since 2010. The net foreign inflow was 38,317 last year, the data showed.

Although home to an aging population, the number of Spanish citizens also rose slightly last year, boosted by foreign inhabitants acquiring citizenship. A total of 114,207 foreign residents became citizens of Spain last year.

More than one in five of them were from Morocco, with Ecuador, Colombia, Bolivia and the Dominican Republic accounting for most of the rest.

The biggest net outflow of foreign residents in Spain came from Romanian, Bulgarian and British citizens. The biggest net influx was from Italy, Venezuela, Ukraine and Honduras.

(Writing by Dave Graham; Editing by Paul Day, Greg Mahlich)