Labour's narrow lead in the Heywood and Middleton byelection was the result of a "collapse" of the Conservative and Lib Dem vote into support for Ukip, the shadow foreign secretary said.
Douglas Alexander told Good Morning Britain: "We actually increased our share of the vote in the seat last night.
"I was elected myself in a byelection just six months after our greatest landslide in 1997 and got a comparable share of the vote to that which Liz got"
Labour will support government plans to join airstrikes against Islamic State (IS) when the House of Commons votes to join America's broad coalition of countries fighting the insurgency in Iraq, Good Morning Britain has heard.
Shadow foreign secretary Douglas Alexander said there was "a very clear legal basis" to the airstrikes.
The shadow foreign secretary has said that there is a legal basis for the UK to join the coalition which is striking Islamic State (IS) in Iraq, but not Syria.
Speaking to Good Morning Britain Douglas Alexander said:
Labour's shadow foreign secretary Douglas Alexander says that "the decision before us is far bigger than party politics" in his opening statement.
He argues that a unified United Kingdom can deliver: "More and better jobs, a National Health Service that can change and develop the needs for our futures and for our future population, pensions that can deliver dignity and decency as our population ages, a strong Scottish Parliament".
Shadow Foreign Secretary Douglas Alexander has said the British public needs reassurance about the number of UK arms being exported to Israel.
The minister said the government must urgently publish a review into its existing UK export licenses to Israel.
In a statement, he said: “The Government must provide assurances that the Consolidated Criteria on arms control is being upheld, which prevents the export of military and dual use equipment which could be used for internal repression, the abuse of human rights or to provoke or prolong armed conflicts.”
Mr Alexander added it was a "source of shame" that the Prime Minister has so far failed to condemn the scale of Palestinian suffering as a result of the Israeli offensive.
Shadow foreign secretary Douglas Alexander has said that after Russia signed the Geneva accord earlier in the month, they have "since failed to act on its commitments within the agreement, and has taken no public steps to help de-escalate the situation in eastern Ukraine".
He said: "Given Russia's continued actions, the G7 is right to now prepare broader sectoral measures in order to increase the economic and financial cost to Russia of its actions to support the destabilisation of eastern Ukraine.
"The priority now must be avoiding an escalation in the region and securing conditions in which upcoming Presidential elections in Ukraine can take place without the risk of further violence."
The UK Independence Party poses a "mortal threat" to the Conservatives' chances of winning the next election, the shadow foreign secretary has told ITV's The Agenda.
Douglas Alexander told host Tom Bradby that he took the threat from Ukip "very seriously" but added: "I think you defeat Ukip is not [by] what David Cameron has done: first of all ignore them, then insult them, then imitate them."
But he admitted he was "certainly not complacent" and that Labour "have got a lot of work to do."
Mr Alexander added: "If you are a party that aspires to be the government you can't just offer anger, you've got to offer answers - so in that sense I don't want to be a replica of Nigel Farage, I want to be a remedy. That means offering different policies that give people confidence."
Tom is also joined by the broadcaster Mariella Frostrup, the Countdown presenter Nick Hewer and the Spectator's Isabel Hardman.
The Agenda with Tom Bradby will be broadcast on ITV at 10.35pm.
Get involved in the debate on Twitter during the show by using the hashtag #theagenda
Shadow foreign secretary Douglas Alexander said there needs to be "deft and frankly difficult diplomacy in the days and weeks ahead" to ensure Russia changes course on the Ukraine crisis.
Speaking on the BBC's Andrew Marr Show, Mr Alexander said he was "very concerned" about the amassing of Russian troops on the Ukrainian border in recent weeks.
"I think this is probably the biggest security challenge that Europe has faced certainly in the last 20 years", he said.
Mr Alexander also warned the President Vladimir Putin "needs to realise that there would be costs and consequences" if Russian troops move into eastern Ukraine.
Suggestions that Labour could enter a coalition with the Liberal Democrats after next year's General Election have been dismissed as "nonsense" by the man in charge of Labour's election strategy.
Douglas Alexander told the Scottish Labour conference that his party should hold the Lib Dems to account for their role in implementing policies such as welfare reform and a rise in tuition fees.
Alexander insisted that next year's election was "quintessentially winnable" for Labour.
"The Liberal Democrats want to pretend that they are the internal opposition to the Conservatives - they're not the internal opposition, they are the enablers of the Conservatives," he said.
"The reason David Cameron is sitting in Downing Street is because Nick Clegg is sitting next to him at the Cabinet table."
An in/out referendum on EU membership under a Labour government is "frankly unlikely", the shadow foreign secretary Douglas Alexander has said.
He said a vote was still "possible" if there was a transfer of powers from Westminster to Brussels.