Labour is leading a plot in the Lords that is expected to inflict an embarrassing defeat on Theresa May’s plans to eradicate the EU rights charter from domestic law after Brexit.

In a sign of the party flexing its muscles in the upper chamber, The Independent understands that Labour – with support from the Liberal Democrats – will force the Government’s hand in a vote on a critical amendment on Monday.

The anticipated defeat would be humiliating for Theresa May and the Brexit Secretary David Davis, who are planning to use the EU Withdrawal Bill – currently progressing through the Commons – to take Britain out of the EU Charter of Fundamental Rights after the UK’s exit from the bloc in March 2019.

It comes as the Government faces pitched battles in Parliament as it attempts to pass multiple pieces of Brexit legislation through the Commons over the next two years.

The Charter sets out a whole range of civil, political and social rights enjoyed by millions of citizens across the EU and covers everything from workers’ social rights to the protection of personal data, but under the Government’s current plans it will not be carried across into UK law after Brexit.

Labour and Liberal Democrat peers believe it will mean losing a key safeguard in terms of data protection and with support from crossbenchers, have the numbers required to push the Government into retaining a key part of the charter in British law.

Supporters of the amendment told The Independent “no amount of scaremongering” by ministers would prevent them from pursuing the vote, adding if the Conservatives believe they “can use Brexit to dilute our rights, they’ll be bitterly disappointed”.

Crossbenchers believed to be backing the amendment on Monday include Lord Kerr, the architect of Article 50, Lord Pannick, who led the legal challenge against the Government over invoking Article 50, and former diplomat Lord Hannay.

The amendment to the Government’s Data Protection Bill in the Lords will essentially replicate the thrust of Article 8 from the Charter concerning data protection into domestic law. It is also significant as the Lords rarely divide at committee stage on a bill – signalling peers’ discontent with Ms May’s administration.

Speaking to The Independent, Lord Stevenson, the Labour peer leading the cross-party amendment, said ministers claim that the Bill will make data protection laws “fit for the digital age” but it would remove a legal guarantee UK citizens currently enjoy – a right to privacy and protection of personal data.

“As a result, the Government is threatening our chances of establishing a post-Brexit legal framework to permit the free-flow of data across European borders,” he said. “If we are unable to reach such an agreement with the EU, there will be no legal basis for the lawful operation of countless British businesses.”

Lord Stevenson continued: “Ministers seem blind to the fact that 11 per cent of global data flows pass through the UK and 75 per cent of that traffic is with the EU. More generally, they are doing nothing to assuage wider fears about an executive power grab across all Brexit related-legislation.

“Despite the Government’s somewhat panicked response this weekend, we will continue to press our case in the Lords for an amendment that will both retain the guarantee that our citizens currently have and enshrine their rights to privacy and protection of personal data in UK law. That is the right thing to do and no amount of scaremongering by ministers will prevent us from pursuing that aim.”

Ed Davey, the home affairs spokesperson for the Liberal Democrats, said the Government “has to stop pretending it can just ignore Parliament”, adding: “This vote will be an important warning sign of things to come. If Conservatives think they can use Brexit to dilute our rights, they’ll be bitterly disappointed.”

Ironically, the Brexit Secretary has previously relied on the Charter in a legal challenge against the coalition Government in 2013. Recalling the event in the Commons recently, Shadow Brexit Secretary Keir Starmer mocked his counterpart and presented documents at the despatch box, showing that Mr Davis’ lawyer had praised the EU charter in a case brought to the EU’s highest court against the snooper’s charter.