(Reuters) -- Greece will activate collective action clauses forcing investors to take part in a bond swap next month at a threshold of 66 percent participation in the process, a government minister who declined to be named said Tuesday.

It will be 66 percent, the minister told Reuters of the threshold level of a debt swap under which private holders will take a write-down of 53.5 percent of the nominal value of their bonds, as part of a 130-billion-euro bailout agreed by euro zone ministers earlier in Brussels.

(Reporting by George Georgiopoulos; writing by Mark John)