Hammersmith MP and shadow justice minister Andy Slaughter flew to America to seek the release of a British citizen in Guantánamo Bay.
The Labour man was part of a cross-party group due to meet with officials in Washington to campaign for the release of Shaker Aamer, who has been locked up at the US military prison for 13 years.
Prime Minister David Cameron has told the US that Britain is willing to take him back - and US Government sources are reported to have said the father-of-four, aged 48, could be released in June.
Ahead of his visit Mr Slaughter said: “It’s time to bring Shaker Aamer home from Guantánamo.”
Mr Aamer is the longest-serving UK resident in Guantanamo.
He was arrested in Jalalabad, Afghanistan, in November 2001 and sent to Guantánamo in February 2002.