Theresa May is poised to make an unprecedented attempt to fix the parliamentary system, allowing her to grab sweeping powers ahead of Brexit, The Independent can reveal.

A late-night Commons vote to secure the Conservatives the muscle to use so-called “Henry VIII powers” to make new laws – behind the backs of MPs – will be staged next week.

The move has been disguised on the Commons order paper under the innocuous description of “motions relating to House business”, but will be a decisive act in the Brexit process.

It will allow the Tories to pack a crucial Commons committee with their own MPs, in defiance of Parliament’s rules, in order to carry out the power grab.

To win the vote, the Conservatives will need the backing of the Democratic Unionist Party (DUP), under the much-criticised “cash-for-votes” deal that props up Ms May in power.

Opposition parties immediately accused the Prime Minister of a bid to “sideline Parliament and grant ministers unprecedented powers” – despite promises to restore sovereignty to MPs.

“This is an unprecedented power grab by a minority government that lost its moral authority as well as its majority at the general election,” Valerie Vaz, Labour’s Shadow Commons Leader, told The Independent.

And Alistair Carmichael, the Liberal Democrat chief whip, said: “The Tories seem determined to ram through their destructive hard Brexit even though they have no mandate for it.”

The bid to seize control of the Committee of Selection comes despite unequivocal advice from parliamentary officials that the Tories must not do so, after losing their Commons majority at the election.

Without the fix, it would be impossible to force through up to 1,000 “corrections” to EU law as intended through the EU (Withdrawal) Bill – the reason for the accusations of a power grab.

Among key rights at stake are protections for British workers and consumers, environmental standards and whether powers will be devolved across the UK, or hoarded by ministers.

Ms May has vowed workers’ rights will be protected, but has declined to say whether that will be enshrined in law – and has made no promise at all about food standards.

Yesterday, ministers admitted a loophole in the legislation could take pension rights away from same-sex couples, unless Westminster acts.

Former Conservative ministers have joined Labour and other parties in protesting that the bill – the key Brexit legislation – will grant ministers unprecedented and dangerous powers.

However, to exploit that power grab, the Government needs control of the committee – which will now be decided in next Tuesday’s showdown vote.

Ms Vaz said: “They will try to sideline opposition in Parliament by rigging the committee system so that they are guaranteed a majority they didn't secure at the ballot box.

“The British people will not understand how having voted to deny the Conservatives a majority, the Tories can alter the rules of Parliament to ensure they have one.”

And Mr Carmichael added: “We will fight tooth and nail to ensure this committee reflects the electorate and does not simply rubber-stamp government decisions.”

The latest controversy comes after one former Conservative minister described the withdrawal legislation as an “astonishing monstrosity” of a bill.

Meanwhile, nearly 40 Tory Eurosceptics were accused of trying to “undermine” the negotiations after signing a letter putting ministers on notice they would accept no backsliding on a hard Brexit.

Abroad, it emerged the Prime Minister has refused an invitation to address the European Parliament, prompting accusations that she is making enemies unnecessarily.

The row centres on the obscure Committee of Selection, which has the crucial task of arranging which statutory instruments (SIs) will be pushed through Parliament and when.

Control of the committee would allow the Government to pass a vast number of SIs – some under “Henry VIII powers”, without proper scrutiny, dating back to a 1539 law.

As revealed by The Independent last month, the Conservatives have previously claimed five of the nine MPs on the committee, but officials have ruled they are entitled to four only, after their Commons majority was destroyed in June.

Despite that advice, Tuesday’s motion will demand those five places – and, therefore, the power to defeat the other parties and control the use of SIs.

The Committee of Selection decides whether the Government has majority on committees for both SIs and primary legislation – without which, ministers are unlikely to press ahead.

The Government has insisted SIs will only be used – sometimes without a vote by MPs – to “correct” EU law where it is necessary to incorporate it successfully onto the UK statute book.

However, they have admitted there are no specific restrictions in the bill to prevent ministers also changing aspects of law they “do not like”.

Moreover, the full extent of the power grab is unknown, because the bill will deliver the power to act over aspects of Brexit where policy will hinge on the outcome of Brussels negotiations.