STOCKHOLM - A group of bondholders in Europe's largest debt collector Intrum is seeking to challenge a restructuring of the Swedish company's own 5.4 billion euro ($5.84 billion) borrowings, the Financial Times reported on Tuesday, citing unnamed sources.

Intrum's share price, which rose last month on the announcement of an agreement with some bondholders, was down 11.75% by 0935 GMT on Tuesday, bringing year-to-date losses to 36%.

The bondholders opposing the deal hold debt that matures next year and were hoping for imminent repayment, making them less inclined to back the restructuring deal than those with longer-maturity bonds, the FT reported.

An Intrum spokesperson declined to comment on the report.

More than half of Intrum's bondholders have signed a binding agreement, the company said earlier this month.

The bond maturing in 2025 accounts for roughly a quarter of the outstanding debt eligible for restructuring, according to Reuters calculations. It is not known what proportion of 2025 bondholders are opposing the deal.

Intrum's restructuring plan - which involves extending maturity dates, reducing some debt and issuing new equity - has led Fitch, S&P, and Moody's to further lower the company's ratings this month.

To implement the deal, Intrum is likely to undergo a corporate restructuring, potentially involving a court process such as U.S. Chapter 11 bankruptcy proceedings or an English scheme of arrangement if it can't gain enough backing for a voluntary rework.

A minimum of 66.7% backing is needed for the U.S. process or 75% for the English one, according to the FT.

Achieving higher levels of consent would simplify the implementation and reduce costs, CEO Andres Rubio said on an earnings call earlier this month, adding that 90% would be optimal, with two-thirds the bare minimum requirement.

On Tuesday, Intrum said it had extended a deadline for bondholders to sign up to the binding agreement to Aug. 19 from July 29. ($1 = 0.9241 euros)

(Reporting by Stine Jacobsen, Marie Mannes and Greta Rosen Fondahn, editing by Terje Solsvik, Kirsten Donovan)