The Foreign Office this week refused to apologise after Baroness Warsi gave Parliament a vastly inflated figure for the number of Palestinian civilians killed by Israel.
After more than 24 hours, a spokesman eventually conceded that the figures were wrong.
One analysis suggested that the false figures were taken from an online organisation backed by the US Campaign to End the Israeli Occupation
But the Foreign Office blamed a “clerical error” for the claim that there had been more than 3,600 fatalities since January 2012.
According to the UN, the actual number was around 260.
The Foreign Office minister was replying to a written Parliamentary question posed by Baroness Tonge, who asked whether the government had a record of the number of “non-violent protestors killed and injured” in the Palestinian territories since 2002. Baroness Warsi said “no assessment” had been recorded.
But she added: “According to the website of the UN Office for the Co-ordination of Humanitarian Affairs, 3,643 Palestinian civilians have been killed by Israelis since 1 January 2012 in incidents directly related to the Israeli/Palestinian conflict in the West Bank and Gaza strip.”
Stuart Polak, director of Conservative Friends of Israel, said the peer’s response to the question had been “deplorable”.
“This official answer from Her Majesty’s Government is at best sloppy and lazy behaviour by the Foreign Office. At worst, it’s a one-sided attempt to slur Israel and Israelis.”
Mr Polak said the Foreign Office should “carefully examine the sources that it relies upon for its information”.
On Wednesday evening, a Foreign Office spokesman said the answer “should have stated 2005 not 2012”.
The Foreign Office offered no apology. Instead, it said Baroness Warsi would write to Baroness Tonge “to correct the response”.
Baroness Warsi has refused to comment.
The exact number of civilian deaths is disputed. B’Tselem, the Israeli Information Center for Human Rights in the Occupied Territories, puts the figure at 518 Palestinians killed by Israeli security forces between 2009 and the end of June this year.
UN figures for 2012 put the number of deaths at around 260, with a further 13 fatalities so far this year.
The numbers presented by Baroness Warsi were originally examined by the Trending Central website, which claimed the 3,643 figure had been obtained from an online organisation backed by the US Campaign to End the Israeli Occupation.
Trending Central also quoted an unnamed Conservative source who claimed Baroness Warsi had “lied” to Parliament — a “serious and dreadful breach of her responsibilities”.
A spokesman at the Israeli Embassy said: “It is vital that the British public has access to accurate information. Unfortunately in this case, grossly inaccurate data was provided”.
Harrow East Conservative MP Bob Blackman said: “It is totally unacceptable that incorrect figures are announced without clarity on the source or basis behind which they have been obtained.”