Labour has hit back in the bitter row over abuse of election candidates, accusing the Conservatives of running a campaign of “smears and untruths” aimed at its leading women.

In an extraordinary hard-hitting letter, the party claims Theresa May’s party “promoted personal attacks as a core component of its national campaign”.

It comes close to accusing the Conservatives of racism by protesting its “negative, nasty campaign” had targeted “the first black woman MP, Diane Abbott”.

The Tories were guilty of abuse on “an industrial scale by spending millions of pounds to post highly personalised and nasty attack adverts voters' Facebook timelines without their permission”, it adds.

The letter has been sent by Ian Lavery, the Labour party chairman, to his Conservative counterpart, Patrick McLaughlin, ahead of a Commons debate on Wednesday.

It comes after Theresa May accused Jeremy Corbyn of failing to condemn threats against Conservative election candidates, revealed in recent days.

The Prime Minister said she was “surprised” that the Labour leader had not spoken out about the mounting evidence of shocking intimidation on the campaign trail last month.

Downing Street is to hold a review of criminal and election law, after Tory MPs including Sarah Wollaston and Sheryll Murray revealed the abuse they had suffered.

Now Labour has upped the stakes with its own counter-accusation, in a letter also signed by Cat Smith, the shadow minister for voter engagement.

It reads: “We are writing to express our dismay and deep concern at the vitriolic personal attacks that defined the Conservative Party’s election campaign.

“The Conservatives’ ran a negative, nasty campaign, propagating personal attacks, smears and untruths, particularly aimed at one of the most prominent women MPs, and indeed the first black woman MP, Diane Abbott.

“Such attacks on politicians, the consequent intimidating and abusive language and threats of violence towards them online, deter many people from entering politics.

“Parties and politicians have a responsibility to set an example, by treating others with dignity and respect, including those with whom we strongly disagree.

“The Conservative Party has instead promoted personal attacks as a core component of its national campaign.”

Arguing it was “not an isolated incident”, the letter also highlights Zac Goldsmith’s “racially discriminatory campaign” against Sadiq Khan, in last year’s race to be London mayor.

The suspension of Anne Marie Morris, the MP who issued a racist slur at a Brexit meeting, was also “evidence of the level to which abusive and discriminatory language has been tolerated”.

Sir Patrick must act by “implementing a zero-tolerance approach to abuse, discrimination and intimidation” following the debate, Labour said.

The letter is likely to see relations between the two major parties sink to a new low – just days after Ms May suggested cross-party working to introduce new sanctions to curb intimidation of prospective MPs.

Last week, Ms Murray revealed that opponents had carved swastikas into her campaign posters and urinated on her office door.

Ms Wollaston, the chairwoman of the Commons Health Select Committee in the last Parliament, said a masked man had covered her office with insults in an attempt to “force me out of town”.

The Prime Minister was then asked if she was concerned that “Jeremy Corbyn has failed to condemn the actions of some of his supporters”.