The West Bank barrier has been highly controversial ever since the Israeli government decided to build it in 2002.
Now it is being challenged in court, both in Israel and at the International Court of Justice in The Hague.
BBC News Online answers questions about the plan.
Wall? Fence? What exactly is this structure?
"The Thing", as one commentator has drolly called it, is in fact part-wall, part-fence. Most of its 700-kilometre (440-mile) length is made up of a concrete base with a five-metre-high wire-and-mesh superstructure. Rolls of razor wire and a four-metre-deep ditch are placed on one side. In addition, the structure is fitted with electronic sensors and has an earth-covered "trace road" beside it where footprints of anyone crossing can be seen.
Parts of the structure consist of an eight-metre-high solid concrete wall, complete with massive watchtowers. The solid section around the Palestinian town of Qalqilya is conceived as a "sniper wall" to prevent gun attacks against Israeli motorists on the nearby Trans-Israel Highway.
Work started in June 2002 and contractors have now completed about a quarter of the planned barrier: a long segment on the north-west edge of the West Bank; two sections either side of Jerusalem; and a section in the Jordan Valley.
But construction has been slowed with the Israelis announcing some changes to the route - for instance around the town of Baka al-Sharqiya, where eight km (5-mile) of fence is to be removed.
Why is Israel building it?
After initial hesitation, the government adopted the plan saying it was essential to prevent Palestinian would-be suicide bombers from entering Israel and attacking Israeli civilians, as has happened many times during the Palestinian intifada.
The initial hesitation can be explained by reluctance among ministers and their hardline supporters to build any structure which might be construed as a future Israeli-Palestinian border which left Jewish settlements stranded in Palestinian land.
Pro-settlement objections have been largely assuaged by the fact that the structure is not being built on Israel's pre-1967 boundary, but snakes several kilometres into the West Bank to link settlements up to Israel.
What are the main objections to the plan?
For Israel's critics, the plan epitomises everything that is wrong with Israel's occupation of Palestinian land and its approach to making peace with its Arab neighbours.
Palestinian land is confiscated to build the barrier; hundreds of Palestinian farmers and traders are cut off from their land and means of economic survival. Most significantly, it creates "facts on the ground" and imposes unilateral solutions which preclude negotiated agreements in the future.
The impact of the plan has been felt most acutely in Qalqilya itself, once known as the West Bank's "fruit basket", which lies within a tight loop in the wall. It is cut off on three sides - from the farms which supply its markets and the region's second-largest water sources in the region. Access to the 40,000-inhabitant town will pass through a single Israeli checkpoint.
Two separate court challenges are being made to the barrier, one in The Hague and the other in Israel itself.
In an Israeli Supreme Court hearing beginning on 9 February 2004, the Hamoked Centre for the Defence of the Individual and the Association for Civil Rights in Israel questioned the principle of building the barrier on occupied land and the restrictions it imposes on the Palestinians in the West Bank.
If the court rules in favour of their petition, correspondents say a lengthy legal process could follow.
Two weeks later, the International Court of Justice in The Hague began hearings into the barrier's legality.
But the court's decision is non-binding, and some countries have argued the ICJ should not be ruling on such a highly-charged case.
Why didn't Israel build it along the old 1967 boundary?
Palestinians say a fence around the entire West Bank might have shown the Israeli government was serious about ending the occupation - the minimum requirement for a fair resolution of the conflict as far as Palestinians are concerned.
As it is, the Palestinians argue, the current plan looks suspiciously like the precursor to a structure which will hem them into 42% of the West Bank - something they believe Mr Sharon has been planning all along.
But Israel argues that the fence is purely a security obstacle, definitely not a part of a future border. Israeli officials argue there is nothing to prevent the fence - erected at a cost of $2m a kilometre - from being moved after a negotiated settlement.
Where does America stand?
Washington, still keen to keep alive the roadmap peace plan, views Israel's "security fence" as problematic because of its capacity to poison the atmosphere between the two sides.
The US has exerted mild pressure on Israel. In July President George W Bush, on the podium with then Palestinian Prime Minister Mahmoud Abbas, said: "It is very difficult to develop confidence between the Palestinians and the Israelis... with a wall snaking through the West Bank."
A few days later, with Mr Sharon, Mr Bush said: "The prime minister made it very clear to me that it was a sensitive issue and my promise to him was we will continue to discuss how to make sure that the fence sends the right signal [to the Palestinians]."
In September, the US raised objections to the proposed extension of the fence. Washington was considering withholding loan guarantees to Israel to the value of the cost of any sections of wall the US considered unnecessary.
But Israel is planning to make some concessions to Washington over the barrier.
Some changes are now likely, amounting to a shortening of the barrier by around 100 km. Plans to loop it around Jewish settlements will be dropped.
The modifications are expected to be presented to US officials due in Israel this month.
What is the UN's position on the barrier?
In late September, the UN issued a report which condemned the barrier as illegal and tantamount to "an unlawful act of annexation".
Then, in December, the General Assembly called on the ICJ to give its opinion.
The resolution was supported by 90 member countries, opposed by eight (Australia, Ethiopia, Israel, Marshall Islands, Micronesia, Nauru, Palau, United States) and 74 abstained.
In his report for the UN Commission on Human Rights, John Dugard, a South African law professor, warned that about 210,000 Palestinians living in the area between the wall and Israel would be cut off from social services, schools and places of work.
"This is likely to lead to a new generation of refugees or internally displaced people," he said.
Israel has dismissed the UN report as "one-sided, highly politicised and biased".
But at the court proceedings in The Hague, 44 countries have filed opinions on the barrier.
Israel, the US, the European Union and others argue that the international court is not the proper forum to discuss the issue.
Another group of countries are urging judges to denounce the barrier as illegal.