Boris Johnson is the new London Mayor

Boris Johnson claimed a remarkable victory in the London mayoral contest on Friday night to cap a disastrous series of results for Gordon Brown in his first electoral test as Prime Minister.

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Newly crowned London Mayor Boris Johnson gives his acceptance speech at City Hall

The Conservative candidate's win over Ken Livingstone followed a calamitous showing for Labour at the local elections - the party's worst performance at the polls for 40 years.

Mr Johnson's landmark victory, a result that would have been almost unthinkable six months ago, was the most symbolic blow to Mr Brown's authority on a day that left the Prime Minister facing the gravest crisis of his leadership.

By taking City Hall, Mr Johnson becomes the first Tory politician to hold a senior role in British politics since the party was swept out of power in 1997. His win provided a significant boost to David Cameron's bid for victory at the next general election.

In the local elections, Labour lost more than 300 councillors and slumped to a humiliating third place behind the Liberal Democrats in the share of the vote – a full 20 points behind Mr Cameron's Conservatives.

The results represented a significant breakthrough for Mr Cameron and were the best the Tories have recorded at the polls since John Major won the 1992 general election. If translated to a General Election, it would see the Tories with a Commons majority of more than 100.

As one Labour backbencher gave Mr Brown six months to reverse the party's slide or resign, comparisons were made between his Premiership and the dying days of John Major's tenure. The Tories predicted the results marked the beginning of the end of Labour's three terms in Government.

On a night of unremittingly bad news for Mr Brown's administration:

• The Conservatives took a 44 per cent share of the national vote, the Liberal Democrats 25 per cent and Labour just 24 per cent;

• Labour lost 331 council seats – far beyond even their worst predictions – while the Tories won 256;

• The Tories made key breakthroughs in the North while Labour was wiped out in the South.

A buoyant Mr Cameron hailed the results as a "very big moment" for the Conservatives, with party strategists likening it to Tony Blair's success in the 1995 local elections, which preceded Labour's landslide election victory two years later.

"I think these results are not just a vote against Gordon Brown and his government," said Mr Cameron, "I think they are a vote of positive confidence in the Conservative Party.

"This is a very big moment for the Conservative Party. I don't want any one to think we would deserve to win an election just on the back of a failing government. I want us to really prove to people that we can make the changes that they want to see."

George Osborne, the shadow chancellor, said the results were a "very, very good springboard".

By 10pm Mr Johnson appeared well on course for victory but he did not manage it on the first preference vote when the result came at midnight.

He was short of the required 50 per cent share, but after the second preference votes were added he was declared winner by 1,168,738 to 1,028,966.

Downing Street attempted to blame its catastrophic losses on the economic downturn. Mr Brown said: "It's clear to me that this has been a disappointing night, indeed a bad night, for Labour. I said I was going to listen and lead.

"We are in difficult economic circumstances. I think people accept that we're going through some of the most challenging times we've seen in many years. The test of leadership is not what happens in a period of success but what happens in difficult circumstances."

If the result were repeated in a general election, the swing would potentially unseat seven Cabinet ministers including Ed Balls, the Schools Secretary, Jacqui Smith, the Home Secretary, and James Purnell, the Work and Pensions Secretary.

Ministers admitted that Labour had been trounced and needed to "listen more" to people's concerns over the economic turmoil.

The decision to abolish the 10p rate of income tax was blamed for much of the reaction against Labour. Mr Brown is now under intense pressure to try to relaunch his faltering government amid allegations that he is out of touch with the concerns of ordinary people.

He is expected to unveil detailed policies this month, including plans to help first-time buyers and people who face having their homes repossessed. There will also be significant constitutional changes. Downing Street aides have indicated that they may rethink proposals to sharply increase car tax.

Mr Balls, one of Mr Brown’s closest colleagues, said: "Voters have been cross with us. They think their tax bills are going up, their fuel bills are going up, their food bills are going up. They need to know we are on their side. We have to do more to show we are delivering for families."

Ian Gibson, a Labour backbencher, set Mr Brown a deadline of this September’s party conference to show that he was the right man to lead Labour into the next election.

He said: "I will give him six months. That takes us up to the autumn conference. If we think that we could lose the next election and we are not moving forward then a lot of people will get rather angry.

"If Brown’s the man to do it then he’s got to give us the right policies. Leaders have to listen to the people below them. It is no use trying to make excuses."

However, despite many Labour MPs privately admitting that they face general election defeat, there appears little prospect of a serious leadership challenge to Mr Brown. Complicated electoral rules and the lack of a heavyweight challenger make a contest unlikely.

The results surprised even the most optimistic Tories. They won control of key target councils in the North, including Bury and North Tyneside.

Mr Cameron said "the Conservative Party is back in Wales" after advances in the Vale of Glamorgan, Monmouth and Cardiff.

The Tories also seized control of Southampton. The last time they controlled the city was in 1984. But they still do not have a significant presence in many northern cities including Liverpool, Manchester, Sheffield and Newcastle.

Labour announced on Friday morning that it had lost Reading, its last council in the South outside London. However, it retained some ground later by winning Slough.

The Liberal Democrats, who gained 34 seats, took control of Hull, once a Labour stronghold. They also held Liverpool, their northern flagship, following the surprise defection of an independent councillor.