LONDON — When the European Parliament bade farewell to Britain last month, its members broke out into a wistful chorus of “Auld Lang Syne.” But when British negotiators travel to Brussels on Monday to kick off the next phase of Brexit, these old acquaintances are likely to give them the cold shoulder.

Negotiations for a sweeping new trade agreement between Britain and the European Union are beginning in an atmosphere of deepening acrimony, with each side accusing the other of bad faith, posturing and moving the goal posts.

Prime Minister Boris Johnson’s government released its objectives for the trade agreement on Thursday — a 30-page document that served mainly to underscore how far apart the two sides are, as they begin a fiendishly complicated negotiation that must produce a deal by the end of the year.

“The whole objective of doing what we’re doing is so the U.K. can do things differently and better,” Mr. Johnson told the BBC, explaining why Britain has rejected a European Union demand that it adhere to European standards on labor, environmental protection and state aid.