"An exact date hasn't been fixed," said a Foreign Office spokesman on Saturday, adding the parties were following a general guideline of late July or early August.
The EU, represented by Britain, France and Germany, were to offer Iran a limited package of nuclear, economic and political incentives to give up work the United States suspects is a veil for efforts to build a nuclear bomb.
In return, the EU wanted Iran to agree to maintain indefinitely its suspension of uranium enrichment, nuclear fuel reprocessing and related activities.
An Iranian nuclear negotiator, Ali Agha Mohammadi, told AFP on Saturday that the so-called EU Three must present their offer for a deal by Monday, which he said was the originally scheduled time, and not a later proposed date of 7 August.
"We reject any delay. We are expecting their proposals to be submitted on 1 August and it should meet Iran's minimum expectations," Mohammadi said.
Mahmoud Ahmadinejad will take
office as president on 3 August
He said the 1 August date had been fixed at a meeting in London and if the Europeans did not stick to this, then Iran would take "measures in line with its national interest".
"It means the proposal regarding Iran's nuclear programme practically will be postponed for an unlimited time," Mohammadi said. "That means this proposal does not meet Iran's minimum expectations."
Iran regards nuclear fuel cycle activities as a right under the Non-Proliferation Treaty and wants to be allowed to keep at least a pilot enrichment programme.
The EU had said on Tuesday that the package would probably be presented in the first week of the month, after Mahmoud
Ahmadinejad takes over the presidency on 3 August.
Asked whether the EU would submit the offer on Monday, the Foreign Office spokesman replied: "I very much doubt it, but I can't confirm the dates."
The EU Three have threatened to back US demands to report Iran to the United Nations Security Council for possible sanctions if it ends its voluntary suspension of enrichment-related activity.