After the result was announced in Kettering, the newly-elected MEP, whose party won 38.23 per cent of the vote, said the Conservatives were paying for Theresa May "betraying" the British people.
The Liberal Democrats also won a seat alongside Labour, but the Tories returned no MEPs after polling just 10.66 per cent of the vote.
She said: "They have been far bigger percentages than I had anticipated, although while I was on the streets in areas as diverse as Boston, Mansfield, Skegness and Chesterfield, the people were telling us they wanted to be listened to.
"Today, they have had their voices heard and we will fight for them tooth and nail."
"Theresa May has betrayed the British people. She made a lot of promises she didn't stick to and unfortunately the whole party is paying for it.
"We are the party that will be listening to them."