David Cameron faced growing Tory pressure on Europe last night when the former cabinet minister Michael Portillo threw his weight behind the call by Lord Lawson for Britain to leave the EU.

As Boris Johnson entered the fray, by saying that Britain should be prepared to quit if it fails to secure better membership terms, Portillo said he "fervently" hopes the British people have the "guts" to vote no.

The intervention by a second Tory grandee came as Charles Moore, Margaret Thatcher's official biographer, revealed that the late prime minister came to believe after she left office that Britain should quit the EU. Writing in this week's issue of the Spectator, Moore says that Thatcher kept her views private after she was advised that her views would push her to "the fringes of public life".

The call this week by Lawson, Thatcher's longest-serving chancellor, for Britain to leave the EU transformed the debate in the Conservative party by providing cover for MPs who felt nervous about speaking out. Portillo today adds fuel to the fire by saying that Britain must leave the EU or risk finding that it has no choice in the future but to join the euro and to enter "political union" with the rest of the EU.

Writing in the Times, which also carried Lawson's article earlier this week, Portillo says: "The referendum, were it to occur, would not be simply about withdrawing from the EU or going on as we are. It would really be about pulling out, or in due course entering political union.

"That is why I would vote no and I fervently hope that the British have more guts than those who govern us, and more than those who govern us think we have."

Portillo says Lawson is wrong to assert that Britain will have a referendum on EU membership. Cameron pledged in his speech on the EU in January that he would hold a referendum by 2017 after negotiating a series of reforms to the EU with fellow European leaders.

But the former defence secretary writes: "I have not been impressed by Mr Cameron's pledge. Given his party's electoral prospects, I doubt if he expects to have to deliver on it. But in any case, he seems to have decided already that Britain should stay in."

The London mayor added to the pressure on Cameron, who claims that his strategy is designed to stabilise Britain's membership in a reformed EU, by saying Britain may have to leave. Johnson told the Times that it was "much less clear" that Britain would suffer if it left.

"The debate is of less existential importance," the mayor said. Johnson added: "But we should be prepared to leave if we cannot get what we want."

The prime minister said he welcomed the intervention by Lawson on the grounds that it highlighted his plans go offer a referendum on a reformed EU. But the renewed debate on Europe within the party comes at an awkward moment for the prime minister. He has been telling fellow EU leaders that his plans are designed to introduce reforms for the EU as a whole. Tory MPs have been told that the prime minister hopes to renegotiate Britain's membership terms.