Labour sources have claimed that one of the party’s eight breakaway MPs tried to access its supporters’ personal data, prompting it to shut down its key canvassing software.

As the general secretary, Jennie Formby, wrote to staff reminding them about their responsibility to guard canvassing information, the Independent Group denied the claims.

A Labour party spokesperson said: “We have become aware of attempts to access personal data held on the party’s systems by individuals who are not authorised to do so. Personal data the party holds about individuals is protected by law, under the GDPR and Data Protection Act 2018.”

Formby told staff in a message that “data held by the party, including data within Contact Creator and other systems used for election or other campaigning work, may only be accessed by individuals who are authorised to access it, and may be used only for purposes authorised by the party as data controller”.

She added: “Much of the data held on our systems tends to reveal individuals’ political opinions and is therefore ‘special category’ data, benefiting from enhanced protection under the legislation.”

But the Independent Group’s Chris Leslie insisted that as he and his colleagues were no longer Labour members they would not seek to use party resources. He accused Labour of “throwing mud” to discredit the newly formed group.

Contact Creator – a key piece of party canvassing software – was shut down on Wednesday, as was another Labour tool, Organise.

In the run-up to the 2017 general election, the party’s campaign coordinator, Andrew Gwynne, told the Guardian: “We can identify specific people in a constituency that candidates will be able to target a message to.”

The latest row underlines the increasingly bitter relationship between the Labour leadership and the eight MPs who have resigned since Monday to sit as independents and have now been joined by three Conservatives.

Earlier, a Labour party source greeted the news of the Conservatives’ resignations by saying: “The Independent Group has now become a Tory-Establishment Coalition. What unites the 11 MPs is their business-as-usual support for austerity, corporate tax cuts and big money corrupting politics.”

Labour is expected to move quickly to select official candidates in the eight constituencies across the country represented by the defectors – and is ramping up the public pressure on them to resign as MPs in order to trigger immediate byelections.

Jeremy Corbyn used a public appearance on Tuesday to remind the MPs that they were elected in 2017 on the party’s manifesto, saying: “They were elected to carry out those policies. They decided to go somewhere else.”

The Labour leader’s tone has been markedly less conciliatory than that of his close ally John McDonnell, who called for a “listening exercise” across the party.

On Tuesday night, the shadow Cabinet Office minister, Jon Trickett, announced that the party would consult on changing the law to make it easier for voters to petition for the removal of MPs who resign from the party they were elected to represent.