David Cameron will miss Prime Minister's Questions as he continues his drive to gain support for a new deal for the UK with the European Union.
The Prime Minister is meeting the leaders of Romania and Poland in the latest round of his diplomatic push to secure agreement on reforms ahead of an in/out referendum on EU membership.
Mr Cameron is locked in a battle with European counterparts over his demand the UK be able to deny migrants in-work benefits for four years - which he says would cut the flow of foreign job-seekers.
He has already been forced to concede that agreement on the UK's proposals will not be reached at next week's EU summit, with a showdown now set for February.
The major stumbling block is Mr Cameron's demand for restrictions on EU migrants' access to benefits, something critics claim is discriminatory and contravenes the rules set by Brussels.
In Romania, Mr Cameron will hold talks with Prime Minister Dacian Ciolos and President Klaus Iohannis before travelling to Warsaw for a working dinner with Prime Minister Beata Szydlo.
Mr Cameron was given a fresh indication of the task facing him in Brussels in a Downing Street meeting on Tuesday with the leader of the liberal grouping in the European Parliament.
In talks about Mr Cameron's demands, Guy Verhofstadt, leader of the Alliance of Liberals and Democrats for Europe, warned that "some areas would be more difficult than others".
A Downing Street spokesman said: "The Prime Minister explained that he wanted Britain to stay in a reformed EU. He outlined the areas where British people had concerns about the status quo, which needed to be as addressed as part of the renegotiation - issues concerning sovereignty, competitiveness, economic governance and migration.
"Mr Verhofstadt said he wanted to engage on the UK proposals on EU reform in a positive, constructive way, while noting that some areas would be more difficult than others."
The measure which is causing Mr Cameron so many problems in the negotiation may end up doing very little to slow immigration to the UK, according to a senior member of the Office for Budget Responsibility.
Giving his personal opinion, economist Sir Stephen Nickell told the Treasury Select Committee: "Changing the benefit rules for EU migration so that they become more difficult to obtain - you are asking me what impact that is likely to have.
"In my opinion: not much."
Sir Stephen, a former member of the Bank of England Monetary Policy Committee, did however say there was "quite a lot of evidence that the differential in job opportunities, the differential in pay, is a significant factor in migration flows ... around the world".
Work and Pensions Secretary Iain Duncan Smith insisted that the door remains open for Britain to secure its welfare demand after European Council president Donald Tusk said there was "no consensus" among the EU's 28 member states on it.
Mr Tusk said on Monday that the other 27 national leaders would need to "hear more" from Mr Cameron before coming to a conclusion on the issue, which he described as the "most delicate" element of the renegotiation of Britain's membership ahead of the in/out referendum promised by 2017.
The comment was widely interpreted as an indication that concessions may be needed from the Prime Minister, who has himself said that he is "open to different ways of dealing with this issue".
But Mr Duncan Smith said that Mr Tusk's letter was "carefully drafted to say that the door is still open" to discussions on Britain's proposal and he was "upbeat" that Mr Cameron will be successful.
Asked what alternatives to the four-year ban he would personally be ready to accept, the Work and Pensions Secretary replied: "That they agree with us. That's an alternative to the way the EU sometimes goes about its business."