The men had refused food and liquids since Monday
One of two Tamil protesters on hunger strike opposite the House of Commons has agreed to suspend his fast.
Sivatharsan Sivakumaraval, 21, and Prarameswaran Subramaniam, 28, said they were prepared to die unless the UK intervened in the war in Sri Lanka.
But Mr Sivakumaraval has suspended his fast for 10 days amid efforts to try to arrange political talks.
Thousands are to march through central London later in protest at Sri Lanka's offensive against the Tamil Tigers.
The two hunger strikers - both students from Mitcham, south London - are part of a group that has been protesting opposite the House of Commons since Monday, over the plight of civilians caught up in the war.
They want Prime Minister Gordon Brown and UN secretary general Ban Ki-moon to agree to meet their representatives.
On Saturday, thousands of people are due to march from Embankment to Green Park in central London in protest at the Sri Lankan government's offensive against the Tamil Tigers and alleged human rights abuses.
Both men agreed to start drinking water after Liberal Democrat MP Simon Hughes said he would try to arrange for a delegation of protesters to visit the UN, Washington and Brussels with Des Browne, the government's special envoy to Sri Lanka, for talks.
Mr Hughes said Mr Sivakumaraval had agreed to accept some food on the condition that he could join Des Browne at the talks, and hoped to speak at a rally after the march.
The MP added that he was negotiating with police to let the protesters continue their occupation for another week.
Mr Hughes said: "Politicians from across the parties in the UK are willing to work with the community here to try to get political movement in the next few days.
"I'm grateful the two young men have agreed to take fluids now while negotiations continue.
"The evidence from Sri Lanka is that we're on the edge of more serious loss of life and real urgency is needed if we are to have a chance of avoiding more bloodshed and the loss of hundreds more lives."
The rebel Tamil Tigers started fighting in the 1970s for a separate state for Tamils in Sri Lanka's north and east. They argue they have been discriminated against by successive Sinhalese-dominated governments.
They are a banned terrorist group in many countries, including the UK.
Thousands of Sri Lankans living in the UK have taken part in the rally that began on Monday afternoon in London's Parliament Square.
They say thousands of civilians in northern Sri Lanka are in danger as its government continues an offensive to "wipe out" the Tamil Tigers.
They are calling for the UK government to act to help end the war and insist they will remain until a ceasefire is called and food and medical aid is allowed to reach civilians.
BBC political correspondent Iain Watson said that although the demonstrations had been going on all week, pressure on the UK government had been increased by the ongoing hunger strikes.
Previously, the pair had signed statements proclaiming that they were on "hunger strike til last breath with full heart" and would not stop until their demands are met.
One of the protesters said they and the British government had a "responsibility" to save the civilians caught up in the war.
The men had said they would take fluids - although not solid foods - if they received assistance from the Labour MP for Mitcham and Morden, Siobhan McDonagh.
The MP, who has a sizeable Tamil community in her constituency, has visited the protests throughout the week.
The demonstrators want her to arrange a trip to the United States to lobby the UN - but she says she has made no firm promises, added our correspondent.
Both hunger strikers said they supported the Tamil Tigers but that their main aim was to achieve a ceasefire.
Mr Browne said he supported a ceasefire and that the British government remained concerned about the plight of civilians caught up in the conflict.
Earlier this week seven people were arrested during clashes with police, who have been trying to bring the protest to an end.
Police say the demonstrators' action is unlawful because they did not give notice of the rally, which initially led to the closure of Westminster Bridge.
Officers have requested that flags bearing images representing the Tamil Tigers be removed.
The United Nations believes up to 150,000 civilians are trapped in northern Sri Lanka and the Tamils claim they are being subjected to human rights abuses.
In recent weeks, the clashes between security forces and Tamil Tigers have intensified as the security forces enter what they say is a final push to defeat the rebels and end nearly three decades of conflict.
The United Nations says more than 2,800 civilians may have been killed and 7,000 others injured in the fighting in the north-east in the last two months. The Sri Lankan government disputes these figures.