By Caroline Valetkevitch

NEW YORK (Reuters) - Currency market fluctuations in the second quarter had the smallest negative impact in two years on North American and European companies' financial results, according to a FIREapps report on Monday.

But exchange rate fluctuations likely had a bigger effect in the third quarter because of Britain's vote to leave the European Union as well as the U.S. presidential race's effect on the currency market, FIREapps CEO Wolfgang Koester said.

Britain's June decision to exit the EU occurred too late in the second quarter for companies to fully quantify the effect, he said.

"We expected it to be a larger impact in Q2, and it didn't show up... I think the numbers are going to come in worse in Q3," he said.

The British pound <GBP=> was mentioned by companies more than any other currency in second-quarter reports, their data showed.

And so far in third-quarter reports, more companies are citing impacts from the Canadian dollar <CAD=> and Mexican peso than other currencies, Koester said, noting that the Mexican peso <MXN=> has strengthened along with Democratic U.S. presidential candidate Hillary Clinton's lead over Republican Donald Trump.

He declined, however, to estimate how much those factors could affect future results.

For the second quarter of this year, the negative impact of currency fluctuations on North American and European companies that quantified an exchange-rate effect was $10.26 billion, the smallest for any quarter since the third quarter of 2014, when the effect was just $8.0 billion, the FIREapps data showed.

For just North American companies, the negative second-quarter impact totaled $6.9 billion, also the lowest since the third quarter of 2014, when the effect was $4.0 billion.

A U.S. dollar index <.DXY> is up 4.6 percent since the Brexit vote.

U.S. multinationals' foreign currency earnings are worth less in dollars when the dollar is stronger.

(Reporting by Caroline Valetkevitch; Editing by Andrew Hay)