A shock result in Britain's snap general election has resulted in a hung parliament in a major blow to Theresa May who wanted to boost the Conservatives majority in parliament.

Instead, her election gamble has failed and she and her Conservative party have been weakened, with many on Friday questioning May's future as prime minister.

"What the country needs more than ever is certainty... it is clear that only the Conservative and Unionist Party has the legitimacy and ability to provide that" May said as she pushed to form a government with the backing of the Democratic Unionists.

But opposition politicians and some members of her own party have called on her to quit.

 "The mandate she's got is lost Conservative seats, lost votes, lost support and lost confidence. I would have thought that's enough to go," main opposition Labour leader Jeremy Corbyn said.

The Conservatives "have lost their majority and the prime minister has lost all authority and credibility,"  said Scottish First Minister Nicola Sturgeon.

"She put her party before her country. She has been found out. She should be ashamed. Theresa May promised strong and stable leadership. She has brought weakness and uncertainty. If she has an ounce of self-respect, she will resign," said Liberal Democrat leader Tim Farron.

Conservative MP and former minister Anna Soubry said May "is in a very difficult place... she now has to obviously consider her position."

"Hard Brexit went in the rubbish bin tonight,"  said former Conservative finance minister George Osborne.

"I hope we never hold referendums on anything ever again," said pro-EU former Conservative finance minister Ken Clarke.