Ministers face a guerrilla war from peers over their plans for reform of the health service – despite winning two crucial votes in the House of Lords.

The Lords voted yesterday to give a second reading for the Health and Social Care Bill and threw out a separate attempt to hold it up. But Andrew Lansley, the Health Secretary, will still have an uphill struggle to get the contentious measure on to the statute book.

Critics of the measures, including Liberal Democrat peers and crossbenchers, have already identified several issues on which they believe they can muster a majority against the Government. Among the potential flashpoints are moves to increase competition, the role of the watchdog Monitor, changes to the structure of the health service and changes to the Health Secretary's constitutional position.

One Labour frontbencher said the Lords authorities should "dust off the camp-beds" in anticipation of late-night sittings during the Bill's committee stage. A former Labour cabinet minister added: "The fight is only just starting."

Peers rejected by 330 to 262 votes an attempt by Lord Owen, the former SDP leader, and Lord Hennessy, the constitutional expert, to refer the Bill to a special committee. The Lords also voted down a bid to kill off the legislation altogether.

By 354 to 220 votes they defeated a call from the Labour peer Lord Rea, a former GP, to refuse the Bill a second reading.

The legislation now moves to the committee stage in the Lords, where it will undergo detailed scrutiny.