Tendayi Achiume said the EU referendum had resulted in a rise in 'racial discrimination and intolerance'.
And she pointedly said that anti-Semitic abuse and attacks surged last year in the aftermath of the Brexit vote.
Labour has been dogged by claims of anti-Semitism ever since Jeremy Corbyn was elected leader in 2016, with Jewish Labour MPs telling how they received death and rape threats after they spoke out.
The Labour leader has vowed to do more to root out the abuse after backbenchers turned on him for allowing racism to fester among some of his supporters.
MPs tore into Ms Achiume's claims that Brexit had made Britain more racist branding them 'complete rubbish' and 'not worth the paper they are written on'.
Labour MP John Mann, chairman of the all party campaign group on anti-Semitism, told Mail Online: 'An ignorant and foolish claim which has literally no basis in fact.
'Quite extraordinary buffoonery.'
The UN inspector has spent the past two weeks travelling the country speaking to victims of racism.
Tendayi Achiume (pictured at a conference last year) said the EU referendum had resulted in a rise in 'racial discrimination and intolerance'
Addressing a press conference in London today, she said suggested Britons are in a state of 'national panic' about terrorism - even though the UK was hit by five terror attacks last year killing dozens and injuring hundreds more.
And she accused the UK of a string of failures which have fostered racism and inequality - singling out the Brexit vote for particular criticism.
She said: 'I think the environment leading up to the referendum, during the referendum and after the referendum has made racial ethnicities more vulnerable to racial discrimination and intolerance.'
The UN envoy who produced the report is a controversial figure who is a connected to a US pressure group that campaigns to abolish prisons.
Zambian-born academic E Tendayi Achiume, 36, is UN special rapporteur on racism and related intolerance.
She has close links to the LA campaign group Dignity and Power, which campaigns to abolish jails.
Four years ago she claimed attacks against foreign nationals threatened the lives of refugees in countries including the UK, comparing it to Libya.
Describing her mission previously, she said: ‘Xenophobic discrimination and intolerance aimed at refugees, migrants and even British racial, religious and ethnic minorities will be an important focus’.
She is the latest in a string of dignitaries sent by various branches of the UN over the past decade who have criticised Britain’s human rights record.
Among them was UN adviser Professor Yves Cabannes joined protesters at Dale Farm in Essex in 2011 to condemn the removal of hundreds of travellers from illegal pitches
Two years later a UN housing rapporteur, Raquel Rolnik, demanded an end to the ‘bedroom tax’ that limited housing benefit for people with unused bedrooms in their homes.
The Brazilian academic, who admitted making an animal sacrifice to Karl Marx during a witchcraft ceremony, said the cut was leaving people hungry.
She stayed in a £300-a-night hotel during her visit to London.
South African feminist Rashida Manjoo, UN special rapporteur on violence against women, attacked Britain’s ‘boys’ club sexist culture in 2014.
Earlier this year another special rapporteur, Canadian Leilani Farha, said Britain had a ‘troubling’ attitude to social housing and suggested that the Grenfell Tower disaster showed that human rights laws were being broken.
She added: 'It is also important to draw attention to the increase in anti-Semitic hate speech and violence that accompanied and followed the referendum.
'In 2017, anti-Semitic incidents reached a record level in the UK, with 1,382 anti-Semitic incidents recorded nationwide by the Community Security Trust.
'This figure represents a 3 per cent increase compared to 2016, and was the highest annual total that the organisation recorded since it began gathering such data in 1984.
'The number of violent anti-Semitic assaults increased by 34 per cent compared to the previous year.'
She said that online hate campaigns tended to single out high profile Jewish women, including MPs, subjecting them to a barrage of hate-filled abuse.
And she also suggested that Britons are in a state of national panic over terrorism - even though the UK was hit by five terror attacks last year killing dozens.
She said: 'In recent years, a series of terrorist attacks by individuals purporting to act in the name of Islam have served as triggers for national panic regarding Britain's security.'
Ms Achiumealso said the Government's adoption of 'sweeping austerity measures' since 2010, which have 'disproportionately' affected ethnic minorities.
She slammed Theresa May's 'hostile environment' policy which she oversaw in a bid to crack down on illegal immigration while Home Secretary.
And she added that the picture for young black boys in Britain 'remains grim and has actually worsened'.
Speaking at a press conference to mark the end of her trip, Ms Achiume said: 'The structural socioeconomic exclusion of racial and ethnic minority communities in the United Kingdom is striking.
'The harsh reality is that race, ethnicity, religion, gender, disability status and related categories all continue to determine the life chances and wellbeing of people in Britain in ways that are unacceptable and in many cases unlawful.
'Austerity measures have been disproportionately detrimental to racial and ethnic minority communities. Unsurprisingly, austerity has had especially pronounced inter-sectional consequences, making women of colour the worst affected.'
But today Tory MPs slammed the UN inspector for claiming Brexit had fuelled racism - and said she had completely misunderstood the vote.
Iain Duncan-Smith, the former leader of the Tory party, told Mail Online: 'It is complete rubbish. These reports are always rubbish.
'Every single one of these UN rapporteur reports are not worth the paper they are written on – they are a total waste of time.
And she also suggested that Britons are in a state of national panic over terrorism - even though the UK was hit by five terror attacks last year killing dozens (pictured, floral tributes left to the victims of the Manchester bombing last May)
'The people who come here have always got an axe to grind.'
Tory MP Peter Bone told Mail Online: 'There is no basis for that - she is just plain wrong.
'I think the person who made this claim that we are somehow more racist now is totally ill informed and doesn't understand Brexit.'
He said that current immigration rules are discriminatory be cause they allow EU nationals to move to Britain freely while hitting other nationalities with strict controls.
He said post Brexit a fair immigration system will be developed it make the system equal for al applicants - no matte where they are from.
Tory MP Jacob Rees-Mogg, leader of the influential backbench group of Brexiteers, the European Research group, said: 'The UN ought to have better things to do than issue tendentious reports abut the UK.'