David Cameron is visiting Spain and Portugal as part of his drive to persuade fellow European leaders to go along with a renegotiation of Britain's EU membership.
But talks with Portuguese prime minister Pedro Passos Coehlo and Spanish PM Mariano Rajoy are likely to be overshadowed by the migration crisis which has seen thousands of Syrians make perilous Mediterranean crossings in the hope of obtaining asylum in Europe.
Labour leadership contender Andy Burnham has warned that Mr Cameron risks losing goodwill from fellow EU leaders for his demands for restrictions on internal migration between member-states if the UK refuses to take a "fair" share of refugees from Syria.
European Commissioner Jean-Claude Juncker is expected next week to call on EU nations to sign up to a plan to resettle 160,000 migrants who have arrived in Italy, Greece and Hungary.
Downing Street said the focus of talks in Lisbon and Madrid will be Britain's renegotiation plans.
The Prime Minister's official spokeswoman said: "This is part of the continued engagement that the Prime Minister started following the election and ahead of the June European Council to talk to European leaders about the reforms he is seeking to address the concerns of the British people.
"We have set out clearly the areas where we are seeking reforms and the June European Council agreed that there should be technical talks on these areas. That work continues."
The spokeswoman said Mr Cameron had already spoken to the Spanish and Portuguese PMs about his plans, but added: "Going to Lisbon and Madrid will provide an opportunity for lengthier and more detailed discussions than they have had since the election on this issue.
A renegotiated relationship will be put to British voters in an in/out referendum by the end of 2017 and formal talks have begun at an official level in Brussels.
Mr Cameron's central demands include measures to stem EU migration to the UK - such as a four-year ban on benefits for new migrants - as well as the removal of the requirement for "ever-closer union" in Europe, a greater role for national parliaments, moves to improve economic competitiveness and new protection for EU states, like Britain, which do not use the euro.
Earlier this week, Downing Street dismissed as "pure speculation" reports that Mr Cameron has downgraded his demands for Britain to be excluded from EU employment laws.
The Financial Times had reported that the PM was now prepared to settle for limited protections for the flexibility of the UK labour market, rather than a full-scale opt-out from employment and social regulations.