The Cross-Party Group on Balanced Migration said a political commitment to reducing net immigration to the levels of the early 1990s - around 40,000, compared to 163,000 in 2008 - was the first step towards avoiding the 70 million figure by 2029.
The group's Declaration on Population, entitled "70 million is too many" was signed by 19 parliamentarians, including former Archbishop of Canterbury Lord Carey, former Commons Speaker Baroness Boothroyd, five Labour MPs and peers, 10 Conservatives and two crossbenchers.
The declaration warned that the current scale of immigration is "unprecedented" and said that failure to stem it "would be seriously damaging to the future harmony of our society".
It accused the major political parties of failing to address intense public concern over the issue, and warned that their silence has provided an opportunity for the far-right British National Party.
The declaration said: "Nearly a million votes by our fellow citizens for an extremist party amount to a danger sign which must not be ignored."