The UK's international reputation has suffered "real damage" as a result of the British National Party gaining their first seats in the European Parliament, politicians, unions and race relations groups said.
Labour MEP for London Claude Moraes said that a threshold had been crossed after the far-right party won two seats.
BNP leader Nick Griffin picked up the seat in the North West of England region and Andrew Brons won a seat in the Yorkshire and Humber region.
Peter Hain, the newly-appointed Welsh Secretary, released a statement via United Against Fascism which read: "It's a shameful stain on Britain that we now have racists and fascists representing our country.
"It is vital that everyone now isolates and confronts the BNP and works with United Against Fascism to defeat them."
At a Unite Against Fascism press conference in Westminster, Mr Moraes went on to acknowledge that the BNP had "jumped upon" Gordon Brown's slogan "British jobs for British workers" during the election.
He explained: "That phrase was jumped upon by the BNP, they are trying increasingly to look at what mainstream politicians say to embarrass the politicians as a way of getting support.
He added: "There is real damage here to Britain because we have never elected fascists in a national election. Fascists in the European Parliament where I sit have long wanted members from Britain to join this transnational group so for those reasons there is deep concern that we have now crossed that threshold."
He said that many people would be viewing Britain as a "nastier" place than they had prior to the election.
The BNP wins came as the party appeared to attract significant numbers of disaffected Labour voters.