No 10 has accused the EU of trying to give the UK a worse trade deal than those offered to the US, Canada and Japan, as the two sides clashed ahead of crucial talks scheduled to take just 40 days.

With the opening round of talks due to start on Monday, Downing Street publicly rejected the EU’s opening offer for a trade deal and said it did not recognise the need for a “level playing field” for competition.

It said Brussels was trying to impose “onerous commitments” that would undermine the UK’s legal autonomy and right to set its own regulations.

“The EU has respected the autonomy of other major economies around the world such as Canada and Japan when signing trade deals with them. We just want the same,” a No 10 spokesman said.

“We agree the UK’s trade with the EU is significant. The US’s [trade] is on the same scale – yet that did not stop the EU being willing to offer the US zero tariffs without the kind of level playing field commitments or the legal oversight they have put in today’s mandate.”

The EU’s mandate stated that any future relationship with the UK should be “underpinned by robust commitments to ensure a level playing field for open and fair competition, given the EU and the UK’s geographic proximity and economic interdependence”. This accords with the political declaration signed by both parties when Brexit took place.

Michel Barnier, the EU’s chief negotiator, also unveiled a compromise position on regulation that falls short of requiring the UK to follow all EU rules in future.

The offer involves Brussels retaining the right to apply tariffs or other sanctions if any divergence between the two sides over time leads to “disruptions of the equal condition of competition”, with EU law being the “reference point”.

However, Downing Street reacted negatively to the suggestion and claimed it did not recognise the need for a “level playing field” between the UK and EU at all, in spite of the goals of the political declaration.

“Level playing field is an EU construct, not a piece of terminology which we use,” Boris Johnson’s official spokesman said.

“We have been very clear on our commitment to discuss open and fair competition as part of negotiations, but we will not accept any demands for the UK to follow EU rules, just as we would not expect the EU to accept UK laws.”

Johnson’s spokesman also accused the EU of reneging on the political declaration by not sticking to its timetable governing talks on financial services and data adequacy.

The first four days of talks are due to start on Monday, with fresh rounds every couple of weeks alternating in location between London and Brussels. The two sides expect 10 rounds of four days, taking the total time set aside to about 40 days before the autumn.

Britain is to set out its own negotiating mandate on Thursday. But with the UK effectively threatening to walk away if Brussels insists on sticking to the same rulebook and legal oversight by European courts, Barnier told reporters on Tuesday the EU would “not conclude an agreement at any price”.

He warned that if there was to be any chance of a deal by the end of the year, there could be no “backtracking” by the UK on past commitments made in the withdrawal agreement and the accompanying political declaration.

“If we want to build up a future relationship in the long term based on trust with the UK, we need to make sure that we have trust in the short term,” he said.

“This is predicated on the proper implementation of the withdrawal agreement that has already been signed and ratified. If you read the political declaration that was signed by Boris Johnson, that was negotiated by him and his team, we are not that far apart. In the political declaration every single word counts.”

He added: “If we want to succeed in the very short period of time that Mr Johnson has chosen, we need to make sure that we don’t start backtracking where we should be making progress.”

Barnier warned that negotiations would be “difficult, perhaps even very difficult” but pointed to the successful negotiation of the withdrawal agreement as a sign that the two sides could end up coming to a deal.

“If you look at the discussion with the UK a couple of years ago on the financial settlement, there were many issues that were controversial,” he said.

Downing Street said the UK would “engage constructively” but Johnson’s No 10 has put less emphasis on achieving a deal than under Theresa May’s administration.

A senior government source said the whole point of Brexit was freeing the UK to make its own laws and “you either get the whole point of Brexit or you don’t”. The source also firmly rejected the idea of allowing any new infrastructure to be built to facilitate any trade checks between Northern Ireland and the rest of the UK.

The main sources of tension in the negotiation will be how far the UK follows the EU’s rulebook and whether the European Court of Justice has any oversight affecting the UK.

However, there is also likely to be disagreement over fishing rights. Brussels’ negotiating mandate demands that the UK continues to allow EU fishing fleets access to British waters, but Britain wants a separate annual agreement on access and quotas.

Another disputed clause in the mandate is the EU’s demand that the UK maintains a ban on chlorinated chicken to protects European meat exports but this would creates a potential obstacle to the UK striking a deal with Donald Trump.

On the recommendation of France, a clause was inserted to insist that both sides maintain “health and product sanitary quality in the food and agriculture sector”.

The paragraph called “Environment and Health” provides a catch-all insurance for the EU that certain methods of food production – including particular pesticides, endocrine disrupters and chlorine washes for poultry – will not be used in the UK.

At the weekend, George Eustice, the new UK environment secretary, refused to guarantee that the government would not allow the importation of chlorine-washed chicken as part of a trade deal with the US.

The EU’s position was backed by the National Farmers’ Union, whose president, Minette Batters, said it would be “morally bankrupt” and “insane” of Johnson to drop the UK’s high farming and food standards in order to strike a US deal.