European Union diplomats have warned Britain that time was fast running out for a Brexit deal, and that it may already be too late to ratify one, as negotiators in Brussels began a last-ditch attempt to avoid a tumultuous exit at the end of December.

EU and UK officials resumed intensive negotiations in Brussels this morning on the future relationship treaty.

Both sides are said to be still far apart on the most difficult issues.

Britain, which left the EU in January, has called on the EU to show "more realism" if there was to be more progress in coming days.

A senior EU official said there may no longer be time to put any trade deal agreed into force.

"It's getting terribly late and may be too late already," said the senior EU official.

"They haven't quite reached where they had hoped to be," a second source, an EU diplomat, said, as talks between the bloc's negotiator Michel Barnier and his British counterpart, David Frost, resumed in Brussels.

The UK's chief negotiator has said he will not be changing the UK position that any deal must be compatible with what he called British sovereignty, and taking back control of its laws, trade and waters.

Mr Frost's tweet that he would not be changing tack was a clear warning that the political upheaval in Downing Street last week, especially chief adviser Dominic Cummings' dramatic departure, did not mean that, with the Vote Leave faction neutered, Britain would be softening its stance in the negotiations.

While the level playing field, governance and fisheries remain the key stumbling blocks, the focus is now on the so-called non-regression clauses.

These would be designed to ensure that when producing goods to be sold in each other's markets, both sides would stick to the same standards on things such as labour and employment law, as well as environmental, climate change and taxation standards.

Minister for Foreign Affairs Simon Coveney earlier said it will be difficult but doable to finalise a Brexit trade deal.

Speaking on RTÉ's Morning Ireland, Mr Coveney said there has not been any success in closing the gap between both sides and until this can be found, there will not be an agreement. 

He said time is running out and while Britain's red lines have changed over the last 12 months, the EU has been remarkably consistent, respectful but also firm on following through on what was agreed in the political declaration.

The European Union's chief negotiator, Michel Barnier, said the EU remains "determined" and "patient" in its bid to strike a post-Brexit trade deal with the UK as crunch talks resumed in Brussels.

He said on Twitter: "With @Europarl--EN & all Member States, we remain determined, patient, respectful.

"We want our future cooperation to be open but fair in all areas."

The EU has insisted on an evolution clause so that both sides do not drift apart on standards over time.

The UK is resisting this, and also the fact that the baseline would be the current standards that the EU applies, and which Britain still applies up until 1 January.

On fisheries, senior figures say that both sides are "miles apart".

It is understood that a joint legal text has been progressed on the non-contentious areas, although key paragraphs have yet to be closed, a reminder that nothing is agreed until everything is agreed.

The text is said to be over 600 pages long, including annexes.

For that reason, both member states, and the European Parliament, who have not yet seen the text, will want as much time to analyse it as possible.

That is why they say the end of this week, or early next week, must be the absolute deadline for an agreement.

Additional reporting: Tony Connelly