Tony Blair faces an embarrassing defeat over House of Lords reform today, with most MPs likely to back proposals for a substantially elected second chamber.

MPs expect Labour backbenchers to defy the Prime Minister's call for an all-appointed House of Lords when both Houses of Parliament vote on the issue tonight.

Reformers believe they can secure a Commons majority in favour of at least 60 per cent elected members, despite last-minute appeals to rally support behind Mr Blair. But peers are expected to vote in favour of an all-appointed House of Lords, leaving the reform process deadlocked before a joint committee draws up proposals for the next stage of reform.

The Commons and Lords are promised a free vote on seven options – an all-elected or all-appointed upper house or a chamber that is 80 per cent, 60 per cent, 50 per cent, 40 per cent or 20 per cent elected.

The Trade and Industry Secretary, Patricia Hewitt, and Charles Clarke, the Education Secretary, will both back a substantially elected House of Lords. Peter Hain, the Welsh Secretary, has also come out in favour of an elected upper house. In a letter to The Independent today, Baroness Kennedy of the Shaws, a Labour peer, calls for a fully or substantially elected second chamber. She argues: "Real reform provides an exciting opportunity to refresh our democracy."

The Tory leader, Iain Duncan Smith, will back an 80 per cent elected upper house, and is likely to be followed by most Conservative backbenchers. Most Liberal Democrat MPs are expected to back a largely elected upper house.