Nasa has boosted its cost estimate of a major telescope project to $8.7 billion, even as lawmakers have threatened to slash the space agency’s budget, a spokesman said.

The James Webb Space Telescope, which aims to replace the Hubble Space telescope with great power and accuracy, would now be ready by October 2018, according to the latest estimates.

“Nasa has completed a JWST re-plan that assumes a revised life-cycle-cost of about $8.7 billion and a launch readiness date of October 2018,” spokesman Trent Perrotto said.

“The $8.7 billion life-cycle cost includes development, launch, and five years of operations and science costs,” he said.

“Any final decisions regarding implementation of the JWST replan will be reflected in the FY 2013 budget request,” which President Barack Obama is to send to Congress next year.

In February, Nasa inspector general Paul Martin told lawmakers that the telescope had gone way over its initial budget of $3.5 billion and was likely to come in at around $6.5 billion.

Nasa has also pushed back its scheduled launch – initally set for 2013 – numerous times.

In July, a Congressional subcommittee voted to axe the JWST project from the US space agency budget altogether, though that decision would have to be approved by the entire House and Senate and signed by Mr Obama for it to take effect.

JWST is an international collaboration grouping Nasa, the European Space Agency and the Canadian Space Agency.