Labour has suffered several defeats over the justice bill
Labour has been defeated twice in the House of Lords in votes over the issues of provocation in murder cases and the handling of military inquests.
Plans to stop accused killers using a partner's infidelity as a defence were defeated by 15 votes as the opposition said juries must be free to decide.
Conservative and Lib Dem peers also joined forces to secure an amendment backing specialist military coroners.
A separate vote on new anti-slavery offences was postponed.
Ministers want to remove the defence of infidelity from a murder trial, saying it was not "acceptable" in the 21st Century.
But they were defeated by 99 to 84 votes on the issue, voted on during a debate on the government's Coroners and Justice Bill.
In seeking to legislate on the matter, Lib Dem peer Lord Thomas of Gresford said ministers were refusing to "trust the good common sense of a British jury".
Separately, an opposition amendment calling for a new post of deputy chief coroner to be created to ensure specialist military coroners are properly trained was passed by 153 votes to 127.
Coroners presiding over military inquests have developed specialist knowledge through the conflicts in Afghanistan and Iraq and this must be retained, its supporters said.
Baroness Butler-Sloss said it was "inappropriate" for all coroners to be trained to deal with such inquests and therefore it was important there was someone with "overall responsibility" to make sure those that did were properly qualified.
"If there isn't someone in charge it is just the sort of thing that could slip through the net," she said.
The issue of military inquests has become politically charged in recent years with the government being forced to drop plans last year to hold some inquests in secret on security grounds.
Ministers said existing provisions in the Coroners Bill were adequate to cover the supply of coroners for military inquests.
But debate on another amendment to the bill calling for the creation of new offences on domestic slavery is set to be delayed until Wednesday.
The amendment has the backing of both main opposition parties, as well as unions and civil liberties groups.
They hope to see two new offences created - one of holding someone in servitude, punishable by up to 14 years in prison, and a second offence of forced labour, punishable by a maximum of seven years in prison.
Campaigners say some migrant workers are being forced to accept low wages and long working hours due to threats from employers.
Ministers have insisted current laws give victims enough protection.