Theresa May sits in her hotel room as she prepares a conference speech. REUTERS/Jack Hill /Pool Senior Conservative MPs have seized on a forgotten Government promise to let Parliament decide the response to any referendum result – insisting it must hold true for Brexit.

Ministers agreed, exactly six years ago, that referendums "cannot be legally binding" – which meant MPs and peers should decide "whether or not to take action" on the verdict given by voters.

The unequivocal statement flies in the face of Theresa May’s repeated insistence that her Government, not Parliament, will decide how to deliver Britain’s withdrawal from the EU.

The Prime Minister is locked in a court battle for the right to use the ancient royal prerogative – legal authority derived from the Crown – to trigger the Article 50 withdrawal process, ignoring Parliament.

But, in October 2010, David Cameron’s Government stated exactly the opposite in a little-noticed response to an inquiry by a House of Lords committee.

That inquiry concluded that "because of the sovereignty of Parliament, referendums cannot be legally binding in the UK, and are therefore advisory".

In response, then-constitutional reform minister Mark Harper stated: "The Government agrees with this recommendation.

"Under the UK’s constitutional arrangements, Parliament must be responsible for deciding whether or not to take action in response to a referendum result."

Nicky Morgan, the former Education Secretary who has called for a vote on Article 50, told The Independent that the document bolstered the case for that vote to take place.

Nicky Morgan, the Former Education Secretary, said that the document bolstered the case for a vote before Article 50 is triggered. ADRIAN DENNIS : Getty

She said: "The referendum was advisory and Parliament has to be responsible for deciding whether or not to take action in response to that result.

"The 2010 Government made that important commitment and – although the complexion of the Government has changed since - the current Prime Minister was a very important part of that Government."

Dominic Grieve, the former Attorney General, said: "It is the case that all referendums are advisory – that’s absolutely, abundantly clear.

"The Government doesn’t have a blank cheque on what model we pursue as we come out of the EU, or our future relationship with the European Union, because the electorate made no pronouncement on that whatsoever."

And Ken Clarke, the veteran former Tory Cabinet minister, said: "Constitutionally, this document must be correct, because referendums never played a part in British politics before Harold Wilson called his European referendum.

"And it’s sensible, because a referendum on a big, broad question tells us nothing about the dozens of issues that have to be tackled by the Government in response to it."

MPs worried about a hard Brexit have stressed their aim is not to derail Brexit, but to ensure proper Parliamentary involvement in the decisions that will follow.

In 2010, the report by the House of Lords Select Committee on the Constitution also stated, on referendum results: "It would be difficult for Parliament to ignore a decisive expression of public opinion."

Asked why the Government had changed its position since 2010, a spokesman for the Prime Minister, said: "Parliament voted by a majority of six to one to hold the referendum – and it is a manifesto commitment of the Conservative Party to deliver on its outcome.

"Triggering Article 50 is therefore something for the Government to deliver."

Under pressure from MPs last week, Ms May said they had numerous opportunities to "discuss, debate, question" her strategy, insisting: "Parliament's going to have every opportunity to debate this issue."