LONDON, Feb 22 (Reuters) - The euro skidded over 1 percent against the dollar and hit an almost-three-year low against the yen on Monday, as fears that a possible British exit from the European Union could damage the euro zone and its single currency.

Sterling was the biggest faller on Monday, sliding almost 2.5 percent against the dollar to a seven-year trough of $1.4057 after popular London Mayor Boris Johnson said he would campaign to leave the EU ahead of a June 23 referendum.

But it was clear that the euro was also suffering, falling to a three-week low of $1.1016 and on track for its biggest one-day fall in three months.

“A Brexit would be bad for sterling, but it would also be bad for the euro,” said Mizuho’s head of hedge fund sales in London, Neil Jones.

“A number of market participants are concerned that the UK leaving would not be a good thing economically for Europe, and also it might cause a political shift in other member countries that favour leaving...which would be a deterrent for investment in the euro.”

Jones added that a risk-on mood across markets was also probably weighing on the single currency, which has tended to be negatively correlated with risk appetite in recent months.

The euro fell almost half a percent against the yen, to 124.71 yen, its weakest since April 2013. Against the pound though, the euro was up 1.4 percent at 78.36 pence .

“We think this is more a European versus non-European issue, rather than sterling versus non-sterling issue,” said Barclays currency strategist Nikolaos Sgouropoulos in London.

“If you have right-wing political parties across Europe polling strongly, and suggesting that they will be closing borders and trying to essentially replicate what the UK has done, then the path for euro/sterling will be very uncertain.” (Reporting By Jemima Kelly, editing by Nigel Stephenson)