Better-than-expected US non-farm payrolls data sparked a rally in the dollar, adding to the British currency's losses.

Sterling is also headed for its ninth consecutive week of losses against the euro. Analysts say the outlook for the currency is worsening.

Government bond yields have dropped sharply in recent weeks as investors poured into government debt markets, betting that global central banks would move to ease policy in the face of stubbornly low inflation and slowing economic momentum.

This week, investors decided the Bank of England will not be immune from the pressure to ease and are now pricing in a rate cut over the next 12 months.

Relatively dovish comments from Governor Mark Carney during the week heightened such expectations.

Added to that, weak purchasing managers' index surveys suggested the British economy may have contracted in the second quarter, underlining economic fragility amid prolonged uncertainty about how and when Britain will leave the European Union.

The UK's delayed departure date is scheduled for Oct. 31.

MUFG analysts said "considerably weaker" than expected growth was "increasing pressure on the BoE to shift to a more dovish policy stance following the lead of other major central banks."

"The darkening UK growth outlook and increasing likelihood of looser BoE policy is reinforcing our dovish outlook for the pound heading into the autumn period," the analysts said.

Sterling dropped 0.6pc to $1.2490, its lowest level since January 3 - the "flash crash" when the pound dropped to $1.2409.

Against the euro, the pound was little changed at 89.77 pence, leaving it down 0.3pc on the week.

Marshall Gittler, analyst at ACLS Global, said GDP data due on Wednesday may chime with the weak purchasing managers' index surveys and confirm the British economy contracted in the second quarter.

"Nonetheless, confirming this fact from the official figures is likely to be negative for sterling," he said.

A Reuters poll published on Friday predicted that sterling would rally substantially if Britain leaves the EU with a deal, but weaken further if no such accord is reached.

Most economists think leaving Britain's largest trading bloc without a deal that would maintain close ties between the two would deliver a significant blow to the UK economy