Top flight shares soared close to an all-time high in afternoon trading as the pound plunged to a 31-year low against the dollar.
Sterling has been hammered after Prime Minister Theresa May pledged to kickstart the Brexit process by the end of March next year, sparking fears the government will strike a deal that leaves Britain out of the single market.
The FTSE 100 Index jumped 126.6 points to 7010, beating last April's record close of 7104, as the pound was almost 15% weaker than before the EU referendum on 23 June, making British stocks cheaper for foreign investors.
If shares remain above this level they will set a new closing mark, the record intraday point for blue chip shares is 7122.
The FTSE 250 Index also rocketed 349.2 points to a record 18532.8.
IG chief market analyst Chris Beauchamp said there was "a distinct feel-good factor among UK investors".
Investors were also cheered by British construction output, which returned to growth in September, buoyed by a rebound in housebuilding as production picked up in the months after the country's Brexit vote.
The closely-watched Markit/CIPS UK Construction Purchasing Managers' Index posted a reading of 52.3 in September, up from 49.2 in August, the first time in four months the sector had reported signs of expansion.
In afternoon trading the biggest risers in the FTSE 100 Index were Provident Financial (+190p to 3302p), Pearson (+43p to 806p), Intertek Group (+158p to 3700p), Micro Focus (+85p to 2289p) and CRH (+99p to 2680p).
There were only four fallers in the FTSE 100 Index who were IAG (-2.2p to 394.7p), Mediclinic International (-3.5p to 920.5p), EasyJet (-3p to 1003p) and Land Securities (-1p to 1057p).
In afternoon trading the biggest risers in the FTSE 250 Index were Evraz (+13.1p to 193.8p), BGEO Group (+207p to 3188p), International Personal Finance (+16p to 287.5p), Fidessa Group (+136p to 2592p) and Aberforth (+55p to 1120p).
The biggest fallers in the FTSE 250 Index were Henderson Group (-4.1p to 266.6p), Phoenix Group (-11p to 873p), Acacia Mining (-2p to 506p), Close Brothers (-5p to 1369p) and Derwent London (-6p to 2584p).