By Jason Douglas and Alex Brittain

LONDON--Bank of England officials are struggling to make sense of a big rise in self-employment in the U.K., a development that may affect when officials choose to lift interest rates.

At its April 9 meeting, Monetary Policy Committee officials agreed that the U.K.'s economic recovery appears to be gaining momentum, and BOE staff expect the economy to have expanded by about 1% in the first quarter of 2014, according to minutes of the meeting published Wednesday. All nine members of the rate-setting panel voted to keep the BOE's benchmark interest rate at a low of 0.5%.

But the minutes record that officials spent some time puzzling over recent trends in Britain's labor market and what they might mean for inflation.

Policy makers led by Gov. Mark Carney are expected to raise the central bank's benchmark interest rate early next year, but officials have said the precise timing of and speed of rate hikes will depend on how quickly spare capacity in the economy is used up.

Officials believe much of this slack is concentrated in the labor market, despite falling unemployment, as many Britons aren't working as many hours as they would like.

One "striking feature" of the labor market data has been a surge self-employment, according to April's minutes. Self-employment rose by 200,000 in the three months to January and accounts for roughly half of all employment gains since 2010.

Officials are unsure how far the rise reflects changing work practices or a "disguised" form of labor market slack, the minutes record.

The MPC's concerns underscore how central banks around the world are grappling with changes in their economies that complicate their interest-rate plans.

Write to Jason Douglas at jason.douglas@wsj.com and Alex Brittain at alex.brittain@wsj.com