EU leaders meeting this week in Brussels are still unsure what Britain wants from Brexit, Irish premier Leo Varadkar said, as the European Parliament's president warned London's financial offer was "peanuts".

"It is quite a difficult negotiation when people who want to leave the European Union in Britain don't really seem to agree among themselves what that actually means," Varadkar told BBC television late Tuesday.

In a separate BBC interview, European Parliament president Antonio Tajani voiced similar frustration -- and warned Britain it must be more "realistic" on its offer of the financial settlement.

Prime Minister Theresa May has promised to maintain Britain's contributions for two years after Brexit in March 2019 to complete the current EU budget period, totalling around 20 billion euros ($24 billion).

"Twenty billion is peanuts. The problem is 50 or 60 (billion euros), this is the real situation," Tajani told the Newsnight programme late Tuesday.

He added: "The UK government is not realistic. We need to put the money on the table, we need our money back as Mrs Thatcher said 30 or 40 years ago. Then it is possible to start the negotiations for the new deal."

EU leaders are meeting in Brussels on Thursday for a two-day summit in which they will decide what progress has been made in the first stage of negotiations on Britain's withdrawal.

London had hoped EU leaders would approve the start of talks on the future trading relationship between the two sides, but this is likely to be postponed until a December summit.

"It's still not clear what the UK actually wants in terms of a new relationship," Varadkar told the Spotlight programme.

"Because on the one hand it seems that the UK wants to have a close trading relationship with Europe just like it has now, but it also seems to want something different.

"And it is very hard for us as European prime ministers to understand exactly what the UK wants the new relationship to look like."

Tajani echoed his comments, and referred to the splits in May's cabinet over Britain's future.

"We want to know what the UK want to do. This is the problem. It is not very clear," he said.

"We (the EU) are united. I don't know where is the unity in the UK, because there are many different positions."