Late Show host Letterman earns an estimated $40m (�22m) per year

As part of the deal Kelly Frank pleaded guilty to lesser charges of theft while the kidnap-related charge was dropped.

Mr Frank was a handyman at Mr Letterman's ranch when he was arrested over the alleged plot, which he denied.

Prosecutor Joe Coble said the deal was made because it would achieve the aim of sending Mr Frank to prison.

'Ransom plot'

Mr Frank pleaded guilty to charges of theft and possessing illegally-killed wildlife, plus a lesser charge of obstruction, at a state district court hearing on Monday.

Mr Frank, who has formally worked on the ranch near Choteau in north-western Montana, was arrested in March.

An acquaintance of his told police that Mr Frank had talked about a plan to kidnap Mr Letterman's 16-month-old son, Harry Joseph, and the child's nanny.

Police were told Mr Frank planned to hold the two for 48 hours and attempt to extort $5m (�2.8m) ransom from Mr Letterman.

Kelly Frank had been working at Letterman's ranch

Mr Coble said he agreed to dismiss the kidnap-related charge of solicitation because he believed the plea agreement accomplished what he wanted.

"Kelly Frank needed to go to prison. This gets that done," he said.

The plea agreement called for a 10-year prison sentence on the theft charge, which accused Mr Frank of overcharging Mr Letterman for painting.

It also called for a six-month sentence on the obstruction charge and five years on the wildlife possession charge.

District Judge Mark Buyske said he would decide whether to accept the plea agreement after reviewing a pre-sentence investigation. Sentencing was due to take place on 13 September.