Ministers are seeking a "bankable" promise of future treaty change in renegotiations ahead of the referendum on Britain's future in the European Union, the Foreign Secretary has said.
European leaders have been told that David Cameron must have a "substantial, binding and irreversible" deal to put to voters when the national poll is held by the end of 2017, according to Philip Hammond.
The Prime Minister earlier this year said he wanted "full-on treaty change" but Downing Street accepts that such a move would be difficult to nail down before the vote.
Instead, registered international agreements could be signed off that would be included in future treaty changes, Mr Hammond said.
"What we will be looking for is bankable promises and commitments that are legally binding," he told BBC Radio 4's Today programme.
"We can have international agreements made between the member states, which are registered as international agreements, legally binding, to be taken into account by courts in making decisions that will then be incorporated into the treaties at the next opportunity when the treaties are next opened.
"But we are very clear a deal, if it's going to be sellable to the British people, has to be substantial, it has to be binding and it has to be irreversible.
"I have said it, the Chancellor has said it, the Prime Minister has said it ad nauseam to our European colleagues."
He added: "The discussions are going well, the discussion at political level and at technical level."