Sterling tumbled below key level $1.20 overnight before staging a slight recovery to $1.206. The pound also fell to a two-month low against the euro to sit at around €1.13.
Britain's currency is set for further volatility this week as traders react to emerging Brexit details.
However, the pound's weakness is helping the FTSE 100 push to new record highs.
The bluechip index hit 7354.14 in Monday trading, as its winning streak looks set to continue.
Ken Odeluga, market analyst at City Index, said: "The PM is scheduled to present a speech that is likely to draw worldwide attention, because for the first time, she will present her vision for a 'truly global Britain'.
"We will all be listening and hoping for answers to several key questions, regarding immigration, the single market, the customs union, The City’s desperate wish to keep hold of ‘passporting’, a possible transitional EU regime, and more.
"Whether or not May answers those questions is almost immaterial from the point of view of sterling volatility—we should expect lots more of it either way.
"Sterling’s implied volatility, which shows the extent to which option traders expect prices to swing, spiked to the highest levels since October on Thursday and Friday, revealing both increased speculative interest and anxiety."
Some experts said the Prime Minister's comments could in fact help the pound rally - if there is enough detail around plans to leave the European Union (EU).
Kathleen Brooks, research director at City Index Direct, said: "There is an outside chance that May’s speech, if it includes details on what will replace single market access, could actually benefit the pound, for three reasons.
"Firstly, key Brexiteer, David Davies, has said that it is likely the government will push for a transitional deal to ensure that access to our European trade partners is not stymied during Brexit negotiations.
"Secondly, the weekend papers also featured comments from the European Union’s lead negotiator who voiced concern about shutting the UK’s financial system out of Europe because of the disruption this could cause to financial markets.
"Lastly, the City’s lobby group dropped its request for financial “passporting” rights at the end of last week, which suggests to us that they may believe that a better option is available down the line."