The leader of the Liberal Democrats in Britain will tomorrow deliver a strong warning of the dangers presented by growing support for the British National Party and the rise in anti-Semitism.

Mr Clegg, whose own mother was held in a Prisoner of War camp in the Second World War, believes anti-Semitism remains a problem in Britain and abroad for both the Jewish community and the nation as a whole. It is particularly so because the country's "collective vigilance" against it has slackened and the battle against it has lost momentum, he believes.

In the annual lecture of the European Institute for the Study of Contemporary Anti-Semitism, he will warn that the problem is "getting worse" and that this "enduring prejudice" continued to mutate and adopt new guises.

He will comment critically on the success of the BNP in the European elections with the recession and the political crisis over MPs' expenses.

"Now, with our economy in tatters and our politics in disrepute, the door has been opened to extremists who thrive on spreading hate," he will say. "Despite our blood stained past huge numbers of voters turned out to support racists and homophobes... anti-Semitism - that very light sleeper - is very clearly awake."

He will also single out Conservative leader David Cameron for criticism for standing "shoulder-to-shoulder" with the likes of Poland's Law and Justice Party, which Mr Clegg describes as a party whose views are "anti-gay, deny climate change, espouse fundamentalist Christianity, and who associated themselves with figures regularly accused of anti-Semitism."

Pledging his party, the most pro-European of the main parties, to reinvigorating the battle against anti-Semitism, he will describe how the imprisonment by the Japanese in Indonesia of his Dutch-born mother during the last war had led her to instill in him and his siblings from a young age the importance of tolerance.

Speaking of his horror at the murder of a security guard outside the Holocaust Memorial Museum in Washington last week, he will say: "I abhor the violence that makes Jews feel unsafe in their own homes; the poisonous threats; the desecration of synagogues and cemeteries. And I also cannot abide the less explicit, lazy acceptance of Jewish stereotypes - the casual anti-Semitism - that has crept back in to everyday, British life. So I share the sense of disappointment felt by many British Jews who believe we should have dealt with this by now."

He will argue that Britain should be a place where no Jewish person feels threatened but that instead, attacks on Jews and on their property are up, not down and the feeling of vulnerability within Jewish communities is spreading, not receding.

Earlier this year, the Community Security Trust reported that the number of anti-Semitic incidents in the UK fell for the second year running in 2008, but that this fall was subsequently overshadowed by an unprecedented increase during the recent Gaza conflict with 250 anti-Semitic incidents reported in four weeks alone, meaning 2009 is likely to become the worst year on record for anti-Semitic attacks on Jewish people and property.

"The paradox of all this is that although anti-Semitism is growing, it is becoming harder to see.....in becoming increasingly commonplace anti-Semitism is becoming better concealed," Mr Clegg will warn.

Citing examples such as throwaway remarks about Jewish wealth and influence, conspiracy theories about September 11 and criticisms of Israeli foreign policy that can cast aspersions on all Jews, he will warn: "We have become careless towards this thinly-veiled prejudice." Such carelessness is "immensely dangerous," he will argue.

"Nearly one million people in Britain felt compelled to vote for a party of fascist thugs," he will say of the recent European elections, "A party of fascist thugs led by a Holocaust denier. Now that the previously unthinkable has happened we cannot afford to be careless any longer. We must be more vigilant towards anti-Semitism than ever."

He will argue for a political plan of action that will bring public spending under control and also lead to massive constitutional reform.

"Extremism festers in the spaces created when people abandon mainstream politics. We need to win them back by cleaning up Parliament and handing back power."

The All-Party Parliamentary Inquiry into Anti-Semitism recent recommended proposals to confront rising anti-Semitism, such as recording criminal attacks on Jews and Jewish property. Mr Clegg will propose going further and removing the burden of monitoring anti-Semitic attacks from the Jewish community's own security trust to the state.

Referring to the difficulty of speaking objectively about Israel without giving succour to those who want to perpetrate anti-Semitism, Mr Clegg, who six months ago criticised Israel's "overwhelming use of force" in Gaza, will mirror the concerns of the Chief Rabbi Dr Jonathan Sacks when he says: "Let us be in no doubt about the link between conflicts involving Israel and the rise of anti-Semitic attacks in Britain."

"In exactly the same way that we must be able to speak candidly about Israel without fear of being labelled anti-Semite, it must be possible to speak out about the actions of Islamist extremists without being accused of Islamophobia," he will say.