Theresa May is to warn the EU not to demand the "unacceptable" in negotiations over Northern Ireland's borders, reports BBC.

The prime minister will set out her case to other European leaders at a summit in Salzburg later.

It comes after EU negotiator Michel Barnier suggested a shift in tone on his controversial "backstop" plan to avoid a hard border.

"We are ready to improve this proposal," he said.

The UK and EU both want to avoid a hard border - meaning any physical infrastructure like cameras or guard posts - but can't agree on how.

Mrs May has previously rejected the EU's "backstop" plan - which involves keeping Northern Ireland aligned with its trading rules - saying it would divide up the UK.

Ahead of the Salzburg summit, Mr Barnier said most checks could take place away from border areas. But this was rejected by Northern Ireland's Democratic Unionist Party which said it "still means a border down the Irish Sea although with different kinds of checks".

After Brexit, the 310 mile border between Northern Ireland and the Republic of Ireland will become the UK's border with the EU.

At the moment, thousands of people cross it every day for both work and pleasure - as do goods, like food and medicines, being delivered across the two countries.

As part of the EU single market and customs union, these products do not need to be checked for customs and standards, but when the UK leaves these two arrangements, this all changes.

Nobody wants a hard border for the checks - in fact the 1998 Good Friday agreement, which helped bring peace to Northern Ireland, got rid of security checks as part of the deal, and police in Northern Ireland have warned reinstating them could make crossings targets for violence.

But there is no agreement between the UK and EU, or between Leavers and Remainers, about the answer. Privately, EU officials say those checks could be carried out by British or EU officials, or by health inspectors rather than customs officials, to "de-dramatise" the border issue.

Mrs May has insisted there needs to be "friction-free movement of goods", with no customs or regulatory checks, between Northern Ireland and the Republic, in order to avoid a hard border there.

There are just over six months to go before the UK is due to leave the EU on 29 March 2019.Negotiations are at a critical stage, with both sides hoping for an agreement on the terms of the UK's withdrawal and future trade relations by the start of November at the latest.