The former head of the Civil Service has thrown his weight behind attempts to let Parliament amend any Brexit deal – and potentially stop Britain crashing out of the EU with no deal at all.

Lord O’Donnell intervened in the House of Lords debate on the Article 50 Bill to protest that MPs and peers were being denied a meaningful vote on the outcome of the negotiations.

The controversy will be one of the main flashpoints as the Bill progresses, with some peers confident the Government can be defeated.

Parliament has been promised a vote before the European Parliament starts its consideration of any Brexit deal, probably in late 2018.

But Theresa May has insisted it will be a ‘take-it-or-leave-it’ choice, insisting she will not go back into talks with EU leaders to seek a better deal if hers is rejected at Westminster.

The UK would then be forced onto World Trade Organisation (WTO) rules, imposing annul tariffs of around £6bn on exporters, research today suggested.

Lord O’Donnell, the Cabinet Secretary to three prime ministers until 2011, said: “The “concession” of a vote which has, as the alternative, departure on WTO rules is no choice at all.

“To be honest, I am very surprised that anyone would think that it was any concession.”

Signalling his intent to vote against the Government, he said: “The UK Parliament should have a serious role in what the deal should cover.

“That is what we in this House should aim for with judicious amendments that help us to achieve a better deal for all in this country.”

Lord O’Donnell also dismissed the Prime Minister’s warnings that her negotiations would be undermined, arguing all EU parliaments will go through a similar process.

“So we would not be putting our negotiators at a disadvantage; we would simply be levelling the playing field,” he told peers.

“This negotiation will be much harder than anything I had to deal with. Believe me, Coalitions and so on are completely straightforward compared with this.”

A second former Whitehall mandarin, Nicholas Macpherson, the former permanent secretary to the Treasury also indicated he will push for changes to bolster Parliamentary oversight.

“I am not an enthusiast for a “neverendum”, but I worry that leaving all further scrutiny to the Great Reform Bill will be to leave it too late,” he said.

A further 110 peers have put themselves forward to speak in the Article 50 Bill today, in a second sitting expected to last until midnight.

However, there are not likely to be any votes on controversies such as the ‘meaningful vote’ and the rights of EU citizens until next week at the earliest.