Jack Straw met Israel's defence minister yesterday to discuss the stalled Middle East peace process.
The foreign secretary conveyed to Shaul Mofaz the government's worries over the legality of the security fence in the West Bank.
The Israeli government insists the fence is there to protect its citizens from Palestinian suicide bombers.
But Palestinians, for their part, say Israel wants to annex more territory for itself.
A Foreign Office spokeswoman said Mr Straw "reiterated our opposition to the building of a fence on Palestinian land which we consider unlawful".
Israel on Thursday declined an invitation to attend a World Court hearing on the future of the security barrier. The hearing, due to begin on February 23rd, was scheduled at the behest of the UN General Assembly.
Mr Straw warned Mr Mofaz over the activities of the Israeli Defence Force in the occupied territories.
The Foreign Office spokesman said: "He [Mr Straw] made clear our concerns about excessive use of force which endangers lives."
Mr Straw and Mr Mofaz are understood to have discussed the arrest of an Israeli soldier, charged with the manslaughter of British peace activist, Tom Hurndall.
Mr Hurndall was shot in the head as he was guiding young children to safety in Rafah, southern Gaza, on April 11th last year.
He was a member of the International Solidarity Movement, a pro-Palestinian body whose activists volunteer to serve as "buffers" between soldiers and civilians.
Mr Mofaz is expected to meet with defence counterpart Geoff Hoon and will fly back to Israel on Sunday.