There is master_account__c lookup(Account), Account(standard field) on Opportunity i am trying to prepopulate both the values with same Account field through custom button.I am able to prepouplate Account field but failed to update master_account__c lookup(Account).

Custom button code: /006/e?&accid={!Account.Id}

VF page for record type selection:

<apex:page standardController="Opportunity" tabStyle="Opportunity"  extensions="OpportunityRecordTypeSelection">
   <apex:sectionHeader title="New Opportunity" subtitle="Select Opportunity Record Type"/>
        <apex:form id="theForm">
            <apex:pageBlock title="Select Opportunity Record Type">
                <apex:pageBlocksection >
                    <apex:pageBlockSectionItem >
                        <apex:outputLabel > RecordType of new record </apex:outputLabel>
                        <apex:selectList size="1"  value="{!selectedRecordType}"  >
                            <apex:selectOptions value="{!RecordTypeList}" />
                        </apex:selectList> 
                    </apex:pageBlockSectionItem>
                 </apex:pageBlocksection>
          
                <apex:pageBlockButtons location="bottom">       
                    <apex:pageBlocksection >
                        <apex:pageBlockSectionItem > 
                            <apex:commandButton action="{!continueToOpportunity}" value="Continue" />
                            <apex:commandButton onclick="CloseTab();return false;" action="{!cancelRec}" value="Cancel"/>
                        </apex:pageBlockSectionItem>
                    </apex:pageBlocksection>
                 </apex:pageBlockButtons>   
               </apex:pageBlock>
        </apex:form>
    </apex:page>

Apex controller:

public class OpportunityRecordTypeSelection
    {
        Opportunity a;
        ApexPages.StandardController controller;
         public Id accid;

        public OpportunityRecordTypeSelection(ApexPages.StandardController controller)
        {
            this.controller = controller;
            a = (Opportunity) Controller.getRecord();
         accid = ApexPages.currentPage().getParameters().get('accid');
            
          }
public PageReference continueToOpportunity() 
        {   
            System.debug('@@@'+selectedRecordType);
            
            if((selectedRecordType =='PM-P Opportunity' || selectedRecordType =='FPD PR Opportunity' || selectedRecordType =='IR3535 EU' ||selectedRecordType )
            {
            
           Id devRecordTypeId = Schema.getGlobalDescribe().get('Opportunity').getDescribe().getRecordTypeInfosByName().get(selectedRecordType).getRecordTypeId(); 
             
                if(accid!=Null)
                {
                PageReference pageRef=new PageReference('/006/e?retURL=%2F006%2Fo&accid='+accid+'&RecordType='+devRecordTypeId+'&ent=Opportunity&Master_Account__c='+accid+'&RecordType='+devRecordTypeId+'&ent=Opportunity');
                pageRef.getParameters().put('nooverride','1');
                pageRef.setRedirect(true);
                return pageRef;
                }
}

Please find the URL i am getting once i clicked on continue.