The visit by opposition Labor party MP Straw comes two months after Britain and Iran named non-resident envoys, restoring ties which were severed in 2011 after the closure of British embassy in Tehran.

Straw, 67, was foreign secretary under prime minister Tony Blair as Britain went to war in Iraq in 2003. He has repeatedly called for engagement with Iran to end the crisis over its nuclear program.

Straw will be accompanied by Conservative former finance minister Norman Lamont, Labour MP Jeremy Corbyn and Conservative lawmaker Ben Wallace, his office said in a statement.

They travelled to Tehran on Monday and were due to return to Britain on Friday.

The British MPs, who are all members of a cross-party British parliamentary group on Iran, had accepted an invitation from the British/ Iran Friendship Group of the Iranian parliament, Straw's office said.

"It is hoped that whilst in Tehran the group will be able to make arrangements for a return delegation from the Majlis (Iranian parliament) to visit the UK," the statement said.

Britain's Foreign Office had been "very helpful in facilitating the visit," it added.

Bilateral relations have improved since President Hassan Rouhani took office in August and Iran signed a landmark deal with major powers in November over its nuclear program.

London and Tehran each named a non-resident charge d'affaires -- one step below ambassador -- in November and the diplomats have since exchanged visits.

NTJ/BA