BRUSSELS (REUTERS) - The EU has hardened its stance ahead of negotiations on a new deal with Britain, which left the bloc at the end of last month, demanding fair competition guarantees that would "stand the test of time", according to a document seen by Reuters on Tuesday (Feb 18).

The draft of the negotiating mandate to be approved by the European Union's 27 member states doubled down from an earlier version on demands that Britain adopt a level-playing field with the bloc on areas from state aid to labour and social standards.

The EU's new position emerged a day after British Prime Minister Boris Johnson's Brexit adviser said London would never be bound by the bloc's rules and this was the whole point of its determination to leave.

"To think that we might accept EU supervision on so-called level playing field issues simply fails to see the point of what we are doing," David Frost said in a lecture in Brussels.

"That isn't a simple negotiating position which might move under pressure - it is the point of the whole project."

Britain left the EU at the end of last month, and the two sides are due to launch negotiations on their future relationship in early March.

They aim to agree by the end of the year a deal that would cover a raft of issues from trade to security to fishing to space and environmental cooperation.

The European Commission is negotiating from the EU side and national ambassadors of the EU27 states are due to discuss their updated mandate for the Brussels-based executive on Wednesday.

It is due to get the final stamp of approval at a ministerial gathering later this month.

Changes from a previous version of the mandate included the strengthening of the so-called level playing field provisions.

"The envisaged partnership should include an ambitious, wide-ranging and balanced economic partnership, in so far as there are sufficient guarantees for a level playing field," it said, adding they needed to "stand the test of time".

The 27 would also demand similar clauses for international maritime transport and other areas, the document showed.

In a jibe at Britain, where the ancient Greek Elgin Marbles are on display at London's British Museum, the draft document was also updated to say: "The Parties should... address issues relating to the return or restitution of unlawfully removed cultural objects to their countries of origin."

An EU diplomat said the line on cultural artefacts was proposed by Greece, with support from Italy.

The 27 also stressed more firmly in the latest mandate that they would be taking on their own any decisions on so-called equivalence that would allow access for British financial services to the bloc's single market.