The plant, with a capacity to produce 330 MW of electricity, is being constructed with China's assistance.
"Pakistan feels that this is illegitimate activity because Jammu and Kashmir is an internationally recognised disputed territory and Britain should be aware of it," Foreign Ministry spokesperson Tasneem Aslam said.
Pakistan on Wednesday night suggested there should be a permanent ceasefire between Indian authorities and militant groups in Jammu and Kashmir.
Pakistan has said the statement is against the well-established norms of interstate relations.
'US queries should be forwarded to the Pakistan government, which would investigate and respond.'
Dr Singh had said that he is committed to a pragmatic solution to Indo-Pak issues.
Pakistan on Monday protested against India's plan to throw open the Siachen glacier to tourists.
The announcement to admit Pakistan in ASEM was made at Helsinki on Monday following the sixth Summit of the organisation.
The US has been mounting pressure on Pakistan to act against terror.
Reports said helicopters attacked a residential compound in Saidgi area near the Pakistan-Afghan border.
Pakistan's foreign office spokeswoman Tasneem Aslam said Manmohan Singh had accepted a longstanding invitation to visit Pakistan though he has never committed to any date.
In December 2005, Pakistani Prime Minister Shaukat Aziz had inaugurated the concrete pouring of the country's third nuclear power plant, in southwest of the national capital.
Aslam said that the allegation about existence of 52 terror training camps on Pakistani soil was 'a figment of imagination.'
The spokesperson acknowledged that the peace process was not moving ahead at the desired pace.
Pakistan in its proposals sent to India identified five crossing points-- Tangdhar-Tithwal, Uri-Chakoti, Uri-Hajipeer, Poonch-Rawlakot and Mendhar-Tattapani.
Pakistan justifies the act saying it may be too onerous for small countries to have to participate in another working group as they have staffing issues.
Pakistan's Premier Nawaz Sharif till late on Friday night gave no indication on attending Monday's swearing-in ceremony of Narendra Modi as prime minister amidst reports that he was facing opposition from hardliners in the establishment.