By Gertrude Chavez-Dreyfuss

NEW YORK (Reuters) - Sterling tumbled on Thursday, posting its largest one-day fall in a month against the dollar, after the Bank of England cut interest rates and restarted bond purchases in a move to mitigate the impact of Britain's vote to exit the European Union.

The dollar, meanwhile, gained against a basket of currencies for a second straight session, as investors continued to balance positions ahead of Friday's crucial U.S. nonfarm payrolls report for July.

The focus, however, remained squarely on the British pound in the wake of the widely-expected BoE decision.

As well as cutting rates to a record-low 0.25 percent from 0.5 percent, the BoE launched two new schemes, one to buy 10 billion pounds of high-grade corporate bonds and another - potentially worth up to 100 billion pounds - to ensure banks keep lending even after the rate cut.

Sterling sank 1.5 percent against the dollar in the first half hour after the decision and as BoE Governor Mark Carney started speaking. It was last down 1.5 percent at $1.3122 <GBP=D4>.

"The BoE therefore delivered on market expectations for rate cuts, over-delivered on quantitative easing, both in terms of size and composition, and sent a more aggressive-than-expected signal on future rate cuts," said Sam Lynton-Brown, FX strategist at BNP Paribas in London.

"Sterling/dollar has weakened in line with our view and we still see scope for further downside in the pair."

The Australian and New Zealand dollars, which have suffered in the past week from worries that central banks globally would not meet market expectations for further policy easing, rose around half a percent against the U.S. dollar.

On the other hand, the dollar, driven to a six-week low after a poor U.S. second-quarter gross domestic product (GDP) reading last week, drew strength from the gains against sterling.

The dollar index gained 0.1 percent to 95.674 <.DXY>, holding above a low of 95.003 touched earlier this week.

Ahead of Friday's U.S. jobs report, fed fund futures have priced in a 12 percent chance the Fed will hike rates at its policy meeting next month, unchanged from Wednesday, according to the CME's FedWatch tool. For the December meeting, futures show a 34 percent probability of a hike, compared with 40 percent late on Wednesday.

BNP's Lynton-Brown thinks the market is underpricing the chances of a rate hike next month.

"The change in (Fed) language around labor market utilisation has lowered the bar to 130,000 the level of non-farm payrolls required for a rate hike."

According to a Reuters poll of economists, nonfarm payrolls likely increased by 180,000 jobs in July after surging 287,000 the prior month.

(Reporting by Gertrude Chavez-Dreyfuss; Editing by Paul Simao and Chris Reese)