Ireland’s Taoiseach has dismissed one of Theresa May’s suggestions aimed at winning parliamentary support for her Brexit deal.

Leo Varadkar said he could not give Britain a legal assurance around a start date for a post-Brexit trade deal.

Such an intervention could help the Prime Minister win over Parliament because it would clarify the date when the Irish border backstop was no longer needed.

Mr Varadkar said some of Mrs May’s proposals made sense and others would be “difficult”.

“For us to make a legal commitment to have a done deal at a particular moment or time, that is not possible because it is not in our gift to deliver that, we cannot promise anything that is not in our power to deliver.”

He said they would begin negotiations on the future relationship as soon as possible once the withdrawal treaty was dealt with.

“We can commit to our best endeavours and say we will work towards a target date (for a trade deal).

“It is not possible to say that we will definitely meet that date.

“The future relationship treaty will be more complicated, it will have to be ratified by 28 member parliaments.”

The backstop, which will come into effect if a wider trade deal between the UK and EU fails to materialise, will see Northern Ireland adhere to a range of EU regulatory rules in order to facilitate free-flowing trade across the border.

The Prime Minister is facing widespread opposition to the measure at Westminster amid claims it will undermine the integrity of the United Kingdom by creating an economic border between Northern Ireland and Great Britain.

Mr Varadkar said the British Government needed to be clear about what Parliament needed to pass the withdrawal treaty, which assurances would be enough, and noted European leaders were not planning any extra summits before next March.

“How can we be sure that it is going to be enough?

“We cannot have someone coming back every couple of weeks looking for something extra, looking for something more.

“We cannot operate international relations in this way.”

In Brussels for the second day of the European Council meeting, Mr Varadkar said he was “very satisfied” with the summit conclusions on Brexit which made clear the withdrawal agreement was not “up for renegotiation”.

The Taoiseach said he still believed a no deal was an “unlikely scenario” despite the EU ramping up preparations for such an eventuality.

“As Europe, we reaffirmed our commitment for the need for a backstop,” he said.

“An open border between Northern Ireland and Ireland can’t be a back door to the single market.

“That’s why European countries also very strongly support backstop. It is not just an Irish issue, it is very much a European issue as well.

“It is very much a case of the European Union being one-for-all and all-for-one.”

Mr Varadkar said the issue could not be deferred to the trade talks post-withdrawal.

“It’s about protecting the peace, keeping the border open, also protecting the single market and making sure we answer this question of the Irish border now, so no side can use it as leverage in the future relationship talks, which we are willing to start as soon as the withdrawal agreement is ratified.”