David Cameron issued a stark warning after anti-austerity party Syriza won the Greek Elections.
The Prime Minister said the victory will "increase economic uncertainty across Europe".
The comments come after Alexis Tsipras' party won 36% of the vote and sent a shockwave throughout Europe as fears over a Greek exit from the European Union (EU) grew.
But, despite holding the largest share of the vote after the elections, the party is projected to fall two seats short of an overall majority.
"The Greek election will increase economic uncertainty across Europe," Cameron said.
"That's why the UK must stick to our plan, delivering security at home."
The euro fell to an 11-year low as stocks fell on Monday (26 January) after the snap election.
The leftists' victory has put Greece on course to clash with international leaders over its debt mountain and repayment terms, after it was bailed out by foreign creditors in the wake of the 2009 eurozone crisis.
The euro slid to $1.1098 after the election results were announced, its lowest level since September 2003. It later recovered to $1.1184, which remained 0.2% lower than a week ago.
Tsipras said the new government would work "for Greece and its people to regain their lost dignity." The era of bending to foreign demands for state budget cuts had ended, he added.
The Greek prime-minister elect is set to form the first eurozone government that opposes austerity conditions imposed on sovereign states by the EU and the International Monetary Fund.