Great Britain ended a 26-year wait for Fed Cup promotion when Katie Boulter fought back to seal their 3-1 win over Kazakhstan and spark jubilant scenes.
The team raced on to the court and hugged Boulter after she beat Zarina Diyas 6-7 (1-7) 6-4 6-1.
The British number two trailed by a set and a break - and needed a hot water bottle on her back during changeovers - but refused to give up.
Johanna Konta had earlier put Britain one win away from World Group II.
Konta's own stunning comeback from 4-1 down in the final set to beat Yulia Putintseva 4-6 6-2 7-5 had given the home crowd in London belief that Britain would finally earn promotion in what was their fifth play-off in eight years.
For a while, it seemed the wait would continue as Boulter lost her grip on the match, having won the opening two games, but when she completed her comeback with an emphatic ace on match point, the celebrations began.
"Ah, finally! I'm ecstatic for the team," said Britain captain Anne Keothavong, whose team paraded around the Copper Box Arena, firing tennis balls into the crowd.
"It was such an incredible effort. We kept putting ourselves in this position. But I really feel that having the home advantage this time around made a big difference."
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Celebratory scenes as Boulter finishes the job
After watching Konta come from 4-1 down in the deciding set to beat Kazakh number one Putintseva, Boulter knew victory in her match would seal the tie for Britain.
Boulter admitted she was "struggling physically" during her defeat by Putintseva on Saturday and although she swiftly took a 2-0 lead in the opening set, she repeatedly sat with a hot water bottle on her back during changeovers.
Diyas - who Konta needed two hours and 38 minutes to beat on Saturday - started to find her groove and responded to take the set to a tie-break.
Boulter said Saturday's dramatic defeat - which came despite her holding three match points - would stay in her mind "for a long time" and she seemed nervous - only picking up one point as Diyas comfortably won the tie-break 7-1.
The Kazakh gained the advantage in the second set too but was seemingly distracted by a car alarm seconds before double-faulting and allowing Boulter to break back at 2-2.
The addition of a few drums in the British crowd - to compete with the noisy Kazakh band that had been providing a soundtrack to the action - built a more intimidating atmosphere and Boulter thrived off their support, breaking again late to win the set.
Diyas and Boulter had already suffered three-set defeats this weekend but the Briton was determined not to lose another.
The 22-year-old British number two was quickly 2-0 up before she held off a break point in the third game of the third set - prompting a standing ovation from the home fans.
A lucky net cord helped bring up break point for Boulter at 3-0 up but Diyas saved it with a thumping serve.
That only delayed Boulter's move to increase her advantage as she turned up the gears to make it 5-1.
Konta, fresh from her victory, joined Heather Watson, Katie Swan and Harriet Dart on court ready to celebrate.
And Boulter, on her third match point, threw her arms up in celebration as the crowd erupted in joy.
"I was so nervous watching Katie on the side of the court," said Konta. "I'm sweating so much. We have been in this position for the last three years in a row.
"I am almost speechless which is not normal. I'm still sweating!"
Konta battles back again
Konta has not done things the easy way in the Fed Cup of late - her past five matches in the women's team tennis competition have gone to three sets.
And so she was always going to be in for a battle against Kazakh number one Putintseva, who fought back from 5-2 down in the third set to win the tie-break against Boulter on Saturday.
World number 38 Putintseva - ranked eight places above Konta - showed no signs of tiredness against the Briton, holding off three break points in her first service game before needing just one to take a 2-1 lead.
Konta, who had started with intensity and good variation, broke back immediately and seemed to feed off a much more vocal home crowd.
When Putintseva broke again to move 5-4 up before serving for the first set, Konta responded by silencing the noisy Kazakh band's drums and trumpet for most of the second set - breaking twice and winning 68% of the total points in a dominant display.
Momentum was with the Briton but the crowd were still wary - they had seen Putintseva come from behind the previous day.
The Kazakh was given a taste of her own medicine, though, as it was Konta who completed a resilient fightback.
Putintseva had shown signs of weakness. She had her blood pressure taken during a medical time-out, while ice was applied to her neck and head while she trailed 3-0 in the second set.
Konta went on to take that set in style but then went 4-1 down in the third - two quick breaks followed a time violation and the Briton showed her frustrations by arguing with the umpire.
She did not give up, though, finally breaking back at the third time of asking at 4-2 and went on to win 16 of the last 19 points to seal her 11th successive Fed Cup singles victory.