Plaid Cymru has blamed the Greens for the two parties failing to work together in the European elections.
Voters in Wales will have eight parties to choose from, with at least four calling for another Brexit referendum.
Plaid candidate Patrick McGuinness says his party had spoken to the Greens about forming a "Remain alliance" but claimed the Greens had "felt unable to take it forward".
The Green Party has said no approaches had been made to it by other parties.
In an article published by Plaid Cymru on Saturday, Mr McGuinness, the party's number three candidate for the European elections, said: "Like many, we had hoped for a formal cross-party Remain alliance.
"Indeed, we had been in conversations with the Green Party to that effect, and were disappointed that they felt unable to take that idea forward with us."
He said it could have been "an effective means of strengthening the Remain vote".
However, Mr McGuinness added that Plaid supporters in England will be encouraged to vote for the Greens.
EU elections: Plaid blames Greens for Remain alliance pact failure
Plaid Cymru has blamed the Greens for the two parties failing to work together in the European elections.
Voters in Wales will have eight parties to choose from, with at least four calling for another Brexit referendum.
Plaid candidate Patrick McGuinness says his party had spoken to the Greens about forming a "Remain alliance" but claimed the Greens had "felt unable to take it forward".
The Green Party has said no approaches had been made to it by other parties.
In an article published by Plaid Cymru on Saturday, Mr McGuinness, the party's number three candidate for the European elections, said: "Like many, we had hoped for a formal cross-party Remain alliance.
"Indeed, we had been in conversations with the Green Party to that effect, and were disappointed that they felt unable to take that idea forward with us."
He said it could have been "an effective means of strengthening the Remain vote".
However, Mr McGuinness added that Plaid supporters in England will be encouraged to vote for the Greens.
Of the eight parties on the ballot paper four - Plaid Cymru, the Welsh Liberal Democrats, the Greens and Change UK - are calling for a further referendum on the EU.
On Friday Liberal Democrat leader Vince Cable said it was "a pity" that fellow Remain-backing Change UK had not agreed with them to run a combined campaign.
It is understood that Plaid has not had discussions with either the Lib Dems or Change UK about the possibility of a pact.
The Green Party has been asked to comment on Mr McGuiness's claims.
However, in a statement earlier in April, the Greens said: "In terms of joint lists, the deadline for formal arrangements passed before the EU Council meeting on 10 April, which granted the 'flextension' that makes fighting the European elections possible.
"As far as informal arrangements are concerned, no party has made any approach to us about them, and claims that we have turned down overtures are untrue, since none have been made."
In 2015, the Wales Green Party accused Plaid Cymru members of harassing them on social media after they rejected a plea to stand down in Plaid target seats at that year's general election.
Plaid is defending a seat in the European election in May, as are the Welsh Conservatives and Welsh Labour.
Nathan Gill, who was elected in 2014 as a UKIP MEP for Wales, is standing this time as a member of the Brexit party.
The European Parliament elections in Wales
There are eight parties fighting for four seats in the planned European elections in May.
Welsh Labour, the Welsh Conservatives, Plaid Cymru, the Welsh Liberal Democrats, UKIP and the Green Party are joined by Change UK and the Brexit Party.
Plaid Cymru has blamed the Greens for the two parties failing to work together in the European elections.EU elections: Plaid blames Greens for Remain alliance pact failure
Voters in Wales will have eight parties to choose from, with at least four calling for another Brexit referendum.
Plaid candidate Patrick McGuinness says his party had spoken to the Greens about forming a "Remain alliance" but claimed the Greens had "felt unable to take it forward".
The Green Party has said no approaches had been made to it by other parties.
In an article published by Plaid Cymru on Saturday, Mr McGuinness, the party's number three candidate for the European elections, said: "Like many, we had hoped for a formal cross-party Remain alliance.
"Indeed, we had been in conversations with the Green Party to that effect, and were disappointed that they felt unable to take that idea forward with us."
He said it could have been "an effective means of strengthening the Remain vote".
However, Mr McGuinness added that Plaid supporters in England will be encouraged to vote for the Greens.
EU elections: Plaid blames Greens for Remain alliance pact failure
Plaid Cymru has blamed the Greens for the two parties failing to work together in the European elections.
Voters in Wales will have eight parties to choose from, with at least four calling for another Brexit referendum.
Plaid candidate Patrick McGuinness says his party had spoken to the Greens about forming a "Remain alliance" but claimed the Greens had "felt unable to take it forward".
The Green Party has said no approaches had been made to it by other parties.
In an article published by Plaid Cymru on Saturday, Mr McGuinness, the party's number three candidate for the European elections, said: "Like many, we had hoped for a formal cross-party Remain alliance.
"Indeed, we had been in conversations with the Green Party to that effect, and were disappointed that they felt unable to take that idea forward with us."
He said it could have been "an effective means of strengthening the Remain vote".
However, Mr McGuinness added that Plaid supporters in England will be encouraged to vote for the Greens.
Of the eight parties on the ballot paper four - Plaid Cymru, the Welsh Liberal Democrats, the Greens and Change UK - are calling for a further referendum on the EU.
On Friday Liberal Democrat leader Vince Cable said it was "a pity" that fellow Remain-backing Change UK had not agreed with them to run a combined campaign.
It is understood that Plaid has not had discussions with either the Lib Dems or Change UK about the possibility of a pact.
The Green Party has been asked to comment on Mr McGuiness's claims.
However, in a statement earlier in April, the Greens said: "In terms of joint lists, the deadline for formal arrangements passed before the EU Council meeting on 10 April, which granted the 'flextension' that makes fighting the European elections possible.
"As far as informal arrangements are concerned, no party has made any approach to us about them, and claims that we have turned down overtures are untrue, since none have been made."
In 2015, the Wales Green Party accused Plaid Cymru members of harassing them on social media after they rejected a plea to stand down in Plaid target seats at that year's general election.
Plaid is defending a seat in the European election in May, as are the Welsh Conservatives and Welsh Labour.
Nathan Gill, who was elected in 2014 as a UKIP MEP for Wales, is standing this time as a member of the Brexit party.
The European Parliament elections in Wales
There are eight parties fighting for four seats in the planned European elections in May.
Welsh Labour, the Welsh Conservatives, Plaid Cymru, the Welsh Liberal Democrats, UKIP and the Green Party are joined by Change UK and the Brexit Party.